Runaway

when a legacy engines takes off it is most likely a loss of signal.  make sure your legacy base has a proper ground, and the wire to the track has not come lose.

or it could be something else.

Actually, that could be an encoder issue on the speed control.  If it's not sensed, the engine keeps speeding up thinking it's not moving yet.  If it was total loss of signal, it shouldn't start at all.

I have a legacy GP7 where the flywheel with the encoder ring came loose on the motor shaft causing a similar problem. The flywheel would slow down causing the loco to overcompensate and increase speed.

I was able to observe the flywheel/encoder ring by removing one of the roof hatches, I think the one for adding a 9 volt. I ran the loco and watched the flywheel/encoder ring slow down and the loco accelerate.

I removed the shell and tightened the set screws on the flywheel and the problem was solved. The set screws were tiny flathead screws, I used a screwdriver from an eyeglass kit. 

If it was a signal issue the loco would drop into conventional the intstant the track is powered and take off.

If it can be addressed and started and it will sit and idle but takes off when the throttle is turned,its likely a speed control issue.

Lastly, I'm assuming your operating it with the legacy system but I did have a TMCC remote where the throttle knob went bad on. The knob would lose its "zero" point and the trains would take off the instant the knob was turned.

I believe engines come up in neutral without a command signal. Unfortunately what most people do when they have a signal issue is toggle the power thus creating a runaway condition. 

Marty, don't they come up in neutral once after running previously in command, but after that they come up going forward from then on.  So, if it had previously come up without a signal, even on another day, it could still immediately take off on the first power-up.