Characteristics of the
Interstate Highway System
The US Interstate Highway System (officially, the Eisenhower System of
Interstate and Defense Highways) was built to a
single national standard. However, a few segments of the system that were
constructed before the system was formally begun were built to slightly
different standards. The deviations from standard are fairly small and
relatively rare. We can thus assume that, for practical purposes, the system
has the following universal characteristics:
- Grades no higher than 6%
- Design speed of 70-80 MPH
- Curves no tighter than can be negotiated at 70-80 MPH in a normal, low-performance automobile of the 1950s
- Limits on the radius of vertical curvature (this prevents abrupt transitions in the grade of the
roadbed) Vertical curve radius in meters = 2 x the design speed in km/hr. So for 160 km/hr (100 MPH), vertical curve = 320 meters.
- Curves designed to 600 m minimum radius; there are known areas where this standard is not met (e.g., Providence, RI, on I-95).
- Lanes 12 feet wide
- Very high standard of road surface smoothness
- Limits on the camber of the road surface
- Axle load limits of (I believe) 10 tons per axle
- Bridges at least 14 feet about the road surface (design standard is 15' but 14' 4" is not unusual)
- Paved 10-foot shoulders adjacent to the outside (slow) lane
- Narrow paved shoulders adjacent to the inside (fast) lane
- Long acceleration and deceleration ramps to allow safe entry and exit
- No crossing traffic (I know of at least one rarely-used rail track that crosses an Interstate)
- No opening bridges (I believe this is violated in a few cases)
- The system is, in general, built to a very high standard, with rock cuts well protected from falling rock, sound bridges, good drainage, and rights-of-way that are fenced off in inhabited areas.
In short, this is a resource of unusual quality.
We have seen that conventional railroad systems suffer from
many serious problems, some of which could be solved but most of which are
intrinsic to the basic design. At the same time, it seems likely that we are
going to replace much of the passenger and freight carriage now conducted by
truck or by plane with rail systems, because of limitations on energy
consumption and land occupation. The cost, if conventional rail were chosen,
would be very high. Suppose, however, that we start with a blank sheet of paper
and design a completely new system, Interstate Rail, based on using what will
be the excess capacity of the Interstate highways system as cars are trucks
make way for more efficient transport.
IR would make use of the inside lanes (in
both directions) on most or all Interstate highways. These lanes run
continuously and mostly do not connect to entrance or exit ramps (there are
enough exceptions to this rule to be troublesome, and some interchanges would
have to be reconstructed at significant cost, but the scope of this problem is limited).
A dedicated rail line could thus be installed running in both directions on
every Interstate highway, without the need to acquire new rights-of-way or to
perform heavy civil work to make these rights-of-way suitable for use by
trains, provided that a new rail system can be developed that can operate
within the constraints imposed by the Interstate highway system.
Proposed Interstate Rail System
Ultimately, the entire Interstate system would be converted to IR.
Alterations to Interstate Highways
to Permit Use for IR
- In order to reduce fuel consumption, the speed limits on the
Interstates (and all other roads) should be lowered to 45 MPH, certainly not
more than 55 MPH. This also reduces kinetic energy and so simplifies the
problem of keeping road accidents from blocking the railroad tracks.
- Existing 4-lane highways become double-track railroads and two lane
highways (probably with passing lanes every few miles, provided by
widening and improving the paving of the shoulder for a distance of about a mile.)
- 6- and 8-lane highways give up only one lane in each direction
- Standard New Jersey Center Dividers ("Jersey Barriers") are used to separate rail and road
traffic and prevent road accidents from impinging on the rail line
- Left-side exits and entrances will have to be removed and relocated to the
right side. Since the capacity of the roadways is being reduced, many
high-capacity interchanges can be replaced with standard cloverleafs, which
have capacity enough to handle the reduced load.
Trains would probably not change from one Interstate to another; passengers
needing to change direction would change trains at stations located at interchanges
where Interstate highways cross. This
condition is not essential, but it eliminates the great difficulty of building
rail junctions at these intersections. It may, however, prove easy to use the
existing left-side entrances and exits as rights-of-way for train junctions in
those places where high-capacity interchanges have been constructed.
Rail System Design
Many elements of the track design are, of course, dependent
on the design of the trains — see the train design section below.
Since the system need not interface with existing railroads,
the track gauge can be arranged to whatever is convenient. The current standard
gauge of 4' 8-1/2" dates from Roman times and was only adopted for railroad
use because it was a reasonable compromise between stability, cost of cross
ties, and ability to negotiate curves. Given that the roadbed to be used is 12’
wide (and train car bodies would be about 10' 6" wide), a track gauge as
wide as 9' could reasonably be adopted. Something around 8' would probably work out well, and because
superelevation need not be limited except perhaps by bridge clearances in some
locations (possibly requiring reduced speeds in these areas), all curves can be
"balanced" for 100 MPH operation. The very low center of gravity of the train
design, and the much wider track gauge, would eliminate any problem of a
stopped train capsizing into the center of the curve (although it might be
quite uncomfortable for passengers on a stopped train; the system should in any
case be designed so that trains hardly ever stop anywhere except stations).
Superelevation would be unrestricted within the limitation
of bridge clearance and should permit operation at 100 MPH through almost any
curve on the Interstate system (entrance and exit ramps excluded). With curves
superelevated for balanced operation at 100 MPH, rail wear would be
held to an irreducible minimum. Since the trains would have no solid axles, the
wide gauge presents no problems with tight curvature —each wheel would run
without slipping or sliding.
Tie-plates would be bolted to the road surface using heavy
expansion bolts. The tie plates are equipped with fine-pitch adjusting screws
that would allow rapid, precise positioning of the rails. These adjustors could in theory permit the
gauge and line of the rails to be adjusted every night by automated maintenance
cars traveling over the line. These
cars would detect out-of-line and out-of-gauge conditions, connect motor drives
momentarily to the adjusting screws, and rapidly correct the rail locations. This could result in a base of rails that
was continuously maintained in almost perfect alignment, allowing a very smooth
ride. The excellent sub-grade of the
Interstate highways (well drained and frost free) should remain stable under
traffic. Continuous welded rail eliminates one of the few remaining sources of
rail irregularities. If switches use the movable frogs pioneered on the French
TGV, then the elevation of the rail heads should be almost perfectly regular.
Wheel loading must be held to values that do not exceed the
design standards for the Interstate highways, which appears to be 10 tons. Trucks impose very high instantaneous loads
on the road surface when they fall off the edge of one concrete section onto
the beginning of the next. Over the years, this has created the familiar
bump-bump-bump rhythm of concrete Interstate highways. The actual amount of the
shift is quite small and it should be possible to overcome this using the screw
adjustors. The absence of this shock
load on smooth track might permit the use of higher tie-plate loadings than 10
tons, a point that requires research. Furthermore, because the rail itself is a
fairly stiff I-beam, rail loads are distributed over two or more tie plates,
probably permitting axle loads as high as 20 tons, again a point that must be
verified by research.
The existing AdTranz EuroTram and the Spanish Talgo trains come the closest of any existing equipment to the designs proposed for IR.
Trucks would not use solid axles; each truck would have four
independently-suspended wheels. The wide track gauge and absence of axles would
permit passenger aisles to pass the trucks without rising much above their
usual level, which could be as little as 8" above the rail heads. Such low
floors greatly simplify the task of providing level-loading
platforms at the stations.
The system design speed would be 100 MPH for both passenger
and freight trains, to allow mixed passenger and freight operation. Freight
trains would operate point-to-point, without the need for passing through
classification yards, eliminating this expense and delay.
Trains would be manufactured almost entirely from aluminum,
to save weight. Aircraft technology would be applied where practical to hold
weight to a minimum.
All freight would be containerized; the containers would be
loaded into tubs in the bodies of the freight cars, which should be sufficient
to retain them without any fastenings. Possibly only 20-foot containers could
be accommodated, but it seems probable that 40-foot and even 45-foot containers
could be accommodated. The use of tubs keeps the bottoms of the containers a
foot or less above rail heads, eliminating problems with the limited 14' bridge
clearance (containers are only about 10' 6' high, often less). This would
even leave clearance for a medium-voltage overhead centenary wire, maybe 6000
Rail cars would not exceed 50 feet in length, to minimize
bending loads and reduce structural weight
Each rail car except the first would have only a single
truck that would carry one end of the car to which it was attached; the other
end of the car is articulated to the next car and its weight carried by that
truck. Close attention will have to be paid to wheel and rail loadings, so that
the bearing capacity of the roadbed is not exceeded.
Trucks would be arranged such that the car bodies run close
to the rail heads, rising up above the trucks. This holds the frontal area of
the train to an irreducible minimum.
The car articulations must accommodate both the minimum
vertical curvature of the Interstate highway system as well as the minimum
horizontal curve. Since the cars are short, the car-to-car articulation is kept
smaller than would otherwise be the case.
Initially, trains would be powered by diesel alternators
located in the first and last cars of each train. Trains would be limited to 30
cars (each about half the weight of a conventional railroad car). Assuming that each car weighs 50 tons, the
power required to propel such a train at 100 MPH up a continuous 6% grade is
48,000 HP (excluding wind drag and rolling resistance, which are not important
factors compared to the work required to ascend the grade). This is actually
excessive and would be difficult to achieve. If speed were reduced to 50 MPH
over 6% grades, power could be reduced to 24,000 HP, which is high but not
ridiculous. Given that grades of even 5% are uncommon, and that 16,000 HP will
maintain 100 MPH over a 2% grade and 33 MPH over 6%, we can assume that 8,000
HP alternators at each end of the train would be adequate. Even these are large
by prevailing standards—very powerful single-unit locomotives are now 6000 HP,
but they are, of course, normally used in combinations of several locomotives.
It might be possible to use a 4000 HP conventional diesel alternator for
standard power generation, supplemented by a lightweight (and relatively
inefficient) 4000 HP standby diesel-fuelled gas turbine alternator for those brief
occasions when more power was required.
Later, the generator cars could be replaced with overhead
power (which could be beefed up on steep grades to supply enough power to allow
the each of the traction motors to run briefly at about 250 HP and propel the
train at 65 MPH up a long 6% grade (motors can typically be run at 200% of
rated power for short periods of time; 10 minutes at 65 MPH and 6% grade gives
a rise of 3500 feet, which is surely higher than is encountered anywhere on the
Interstate system; it should be no problem to obtain motors that have a 200%
rating for 10 minutes). If advanced diesel engines are used with low-sulfur
fuel, and provided that the injectors are diligently maintained, exhaust ought
to be quite clean. The problem of PAH particulates needs attention, but
techniques are being developed to remove PAH from diesel exhaust. These engines
could, of course, be run on bio-diesel, if a large enough supply can eventually
An alternative to overhead power is hydrogen-fuel cells,
although the feasibility of this is still questionable. It could give the clean
operation of overhead electrical power together with the very high output
required to power trains up steep grades at high speed.
Both axles on each car would be powered at 260 HP, with each
wheel having its own 130 HP motor ("traction motor") which, taken together,
would absorb the full 16,000 HP generated by the head-end and tail-end power
units. Each motor would be individually controlled by its own
computer to prevent slip and slide.
A more advanced system than the century-old track circuit
should be developed. While highly reliable (if not absolutely perfectly so),
the track circuit required currents of hundreds of amperes to flow over each
one-mile signal block. Even though the voltage is just a few volts, the current
is high enough to require very thick cables to connect the signal relays with
the rails. This approach also requires insulated rail joints every mile, which are
an appreciable maintenance issue and which also cause a rough rail junction
every 35 seconds or so (at 100 MPH). I believe that a combination of inertial
navigation systems, GPS, ground reference, and active satellites could provide
dual-redundant signaling that was at least as reliable as tried-and-true
track-circuit signaling, and at a fraction of the cost. This requires
investigation, of course, and if it proves unfeasible, conventional signaling
techniques would certainly work. Whatever system might be adopted, only cab
signals would be installed. At high speeds, track signals are difficult to
read, and the only safe approach is cab signaling, which is now in widespread
use. Whatever system is adopted should not permit an engineer to operate a train
in violation of a signal. This one measure eliminates about half of all
Stations would be built at existing overpasses, with the
boarding platforms extending in one direction from the bridge and completely
occupying the median area between the two tracks. (This makes it easy to
connect rail stations to city centers using conventional street-running trams,
which can be constructed quickly and cheaply, as they were in large numbers in
the late 19th and early 20th centuries.)
Junctions would exist wherever Interstate highways
intersect. In most cases, the trains would cross and continue along their
routes. Major stations would, of course, be constructed at these locations, as
there would be a considerable amount of connecting traffic. It may be possible
to use existing interchanges to switch trains from one Interstate to another,
but this would mostly be unnecessary.
It is not clear that the standards for vertical curvature on
the Interstate highways are sufficiently stringent that trains could be
operated over these vertical curves at 100 MPH. However, my experience is that
the Interstates have quite gentle vertical curvature and that this would pose
no difficulties. It would require early consideration, however.
The 14' vertical clearance is a significant limitation, but
I believe that the system could be designed around it.
Axle loadings may be the most serious limitation, and it
might be that only 20 ton containers could be transported over the
road, with all heavier containers being shipping by standard railroads. This
would be a significant limitation that might even make it necessary to revert
to the trucks-at-each-end design now used on freight railcars, and
this would almost surely permit the carriage of 40-ton containers. (The
Interstate highways see intense use by trucks weighing 40 tons.)
As with any highway or railroad, snow clearance can be a
significant issue. In territory where heavy snowfall occurs most winters,
snow-clearing trains would have to be equipped with powerful snow blowers
("rotary plows") that would throw the snow far off the rail- and roadway. This
technology has existed for more than a century, and such plows keep
roads and railroads across the Sierra Nevada open except during the most
intense winter storms, when the roadway may close briefly.
We foresee the development of a new rail technology that
could be cheaply and quickly installed on existing Interstate rights-of-way, in
rapid response to suddenly worsening oil supply conditions. The principal
construction material required is the roughly 215 tons of rail needed for each
mile of track constructed plus comparatively small amounts of steel for tie
plates and bolts to fasten the tie plates to the road surface. By far the
largest amount of the work has already been done - the construction of the
Interstate highways. IR merely enables their continued utility under
energy-limited conditions. Many technical questions remain to be answered or
at least verified, but there do not appear to be any fundamental technical
Copyright © 2001 J. Crawford. This page may be freely
reproduced provided that J.H. Crawford is acknowledged as the author, his
copyright is maintained, and the full text is reproduced.
By publication of this work, J. Crawford places in the
public domain any original ideas contained in the work.