that's an interesting situation you're describing, and I don't think that I've got an answer for your question.  The most puzzling part of your statement to me was the part about the flat rail.  My limited understanding of how trains stay on their tracks is summarized thus:

The wheels are tapered (or rounded), such that the outside has a smaller diameter than the inside, and are fixed in pairs to axles.  If one wheel tries to get ahead of another wheel (as though the bogey is trying to steer off of the tracks), the forward wheel starts to ride on a smaller radius, and the lagging wheel starts to ride on a larger radius.  Since the two wheels turn at the same speed (fixed to axle), the forward (outside of turn) one begins to travel more slowly than the lagging (inside of turn) one, and they return to travelling straight-ahead.  The tracks are rounded to meet the wheels appropriately (I don't know the details of the track shape).

I wonder how a train would stay on flat tracks?

I believe ivymike is correct that the wheel taper and live axle are what keeps a conventional train on the tracks.  The flanges don't normally contact the rail.  If you take away either the wheel taper or live axle, you will need some alternate method to keep the train centered on the tracks.  How do you plan to keep this on the tracks?

Roller coasters use another set of wheels running sideways on the edge of their rails, but they carry very light loads and have polyurethane tires on their wheels.  They also don't go near 250 mph.

Thanks guys, your both right; however, it still doesn't give me an answer to the question.

Easier to change a wheel bearing then a solid connected bushing at that weight.
luck
rentapen

Rentapen - Conventional train axles have the bearing outside the wheel, where it is actually pretty easy to change (as large bearings go).  The bolster is lifted off the axle, a cover plate is removed, and the bearing is pulled off the axle.  A picture of one can be found at:
http://www.timken.com/products/bearings/products/ap/Ap-Sp.asp

I can see a number of advantages with fixed independent axels. A lighter assembly. More clearance under your load deck, or in this case you could lower your deck to near rail level. Independent rotation allowing your wheels to turn at different speeds on a curve. Easier to dissipate heat from a wheel bearing. Independent suspension. No fatigue from axel bending moments.

Hush - All of your points are completely valid.  How will the train be kept on the tracks with independant axles?

I'm making the assumption that macetransit has some other mechanism in mind or that he is asking from a purely theoretical point of view. Don't some light rail transit designs use rubber tires on independent axels?

Butelja,
But all road vehicles use the inner free wheeling method. Which is limited to spreading the forces over a larger area then would be needed in a solid configuration whose area would be smaller. If you have the room go for a free turning.
rentapen
toledo, ohio
train capital of the world.

How do the trains with rubber tires stay on the tracks? They do not have the tapper to realign the axle.

What determins the choice of steel vrs rubber whells? I would guess that rubber is better where you need to make lots of stops and starts and steel is better for long runs.

ProEpro

Worked on the Sky bus for Westinghouse, ca 1970. Check out the Tampa International Airport terminal to gate transports. These use pneumatic tires in a slow speed system. They stay on the track because there is a guide beam in the middle. No turning allowed!
Rubber does not work well as a wheel in the solid form.
A pneumatic tire for todays technology would be limited in pressure for a real world operation. This is going to generate a lot of heat at 250 mph.
Sure, land speed racers run those speeds and many applications use high pressure tires, but they are not expected to last long.

Also consider the rolling friction and rotating air friction of a rubber tire versus a steel wheel. You would need a couple of extra engines to turn all that mass.

'Course, someone will decide this is just a design challenge and they are going that way anyhow.

It is not just starting and stopping. Steel-wheel-steel-rail has poor grade climbing ability, as well.

Stick with the proven solid axle tapered wheel design for high speed transport.

Crashj 'big wheels keep on rolling' Johnson

I would think that when 200,000lbs is traveling at
250 m.p.h. brakes would be a concern. What ever this
thing is I'd like to see it go.

Thanks again for the info guys.  Yes it is a High Speed Train with a different type of guidance mechanism.  So it can run on non tapered wheels.  Duel  bearings in each wheel is the way to go then.  Butelja-thanks for the Timken site.  Hush rubber tires on independent axles would probable fly apart at 250mph carrying that kind of weight.  ProEpro trains with rubber tires have a magnetic guidance system, or side wheels to guide them at low speeds.  SnowCrash land speed racers mostly use aluminum alloy wheels. As far as grade climbing ability goes, Linear Accelerators will be used, and sbc A twin-car-set that carries 180 people would weight about 200,000 lbs., but a set of twins would be 12 cars long, all traveling at 250mph.  Linear Induction breaks as well as disks would be used.