Looking For Hints wrt Havoc4 & Trains

I've been working on a very simple one-car train system for one of my sims. The sim in question was one that I got into the havoc4 early adopter program. In havoc4 the train car refuses to stay on the tracks... and I am wondering if this is a problem that can be solved through scripting? I've spent about 3 hours on this but I'm stumped.

The car works great on havoc1. The car has no smarts in it, and no steering. It just uses the linear motor to go either forwards or backwards, until it detects a collision with an object that has the right name (end of the line). Then it waits a minute or two then starts up with the motor in reverse. The track works very simply, there is a full alpha 'rail' that runs along the middle of the rail bed, it's about 0.3m high and 0.1m wide. The rail car has full alpha 'bumpers' inbetween the wheels which follow the guide rail.

In havoc4 though, the car constantly 'bounces' upwards and off the guide rail, which lets the car 'loose' and it wanders till it gets stuck or goes off world.

I've tried a number of things, but nothing seems to be able to eliminate the bouncing:
* I've set the hover function to hover terrain, hover height 0.01, hover efficiency 1.0 and timescale 0.1 -- doesn't prevent or even dampen the bouncing.
* I've set the vehicle buoyancy to -1.0 and that doesn't prevent or even dampen the bouncing.
* I've added a large downward force with the linear motor, so while the train is moving along the X axis, there is a large force on the Z axis, i.e. <5.0, 0.0, -20.0> in an attempt to push the car downwards. This does not have any affect on the bouncing.

In each of the above cases, the car still bounces a lot, and invariably bounces up off the guide rail. All of those settings seem to be in effect 90% of the time, but the 10% of the time it's bouncing, it seems those settings are suspended. Like maybe the physics engine itself is making the bounces happen, to solve some other problems.

I have searched the JIRA and found one open issue re. trains in havoc4, so I voted and commented on it. I also saw a lot of open issues about land vehicles having problems in general, including bouncing or getting hung up on prim seams.

Are there any hints or suggestions I could use? Or is the whole concept of trains a flawed one in SL?

Thanks!

-Atashi

* I've seen conflicting info re. the vehicle buoyancy values. Some notes/wikis say it is a slider between 0.0 (full gravity) and 1.0 (no gravity) and others say it goes from -1.0 (double gravity) to 1.0 (no gravity). Does anyone know definitively if setting it to -1.0 causes more gravity than setting it to 0.0?