Cab2

Unhook the unit from the layout and connect it to a small section of test track and try again.  I would also guess that there is a connection problem because these things aren't known for getting fried....

Also, you may want to reset the ID on your engine because if the engine is set ID 01 (default setting) and you are trying to address it with some other ID number, the engine will just sit there.

When you say Conventional mode, do you mean the engines just stand there, or do they take off running?

The only way for them to be in conventional mode is that there is no base signal present. Even if it was the wrong channel, if there is a TMCC signal it will prevent them from taking off

no, the channel would make no difference if the TMCC signal is present. As long as the TMCC signal is there the engine will sit.

Have you tried a different engine? One of the symptoms of a bad board inside the engine is it just taking off when power is applied. It could be loose or malfunctioning

Had same problem had to send cab2 and base back to Lionel for service

While it could be the bases, I would check it against a second engine first. If the 2nd engine behaves the same you should get the base checked out by Lionel

All of my legacy engines just took off w/ the xfmr set at 18 volts

mention that you are a veteran if you are when you call and shipping is on lionel both ways

Legacy should work from around 12V to 20V on the track. 

Have you tried reinstalling the software in the base and cab2?

Do you have both the transformer(s) and the Legacy unit on the same power switch?

I am wondering if the locomotives are receiving power before the Legacy system has had time to boot up.  Have you tried turning on the Legacy unit, waiting a few seconds until it has finished booting up, and then supplying power to your transformer(s)?

Yes, as Nicole says be sure that you power the base before you power the track.  The engine must see the legacy base first.

doug

FWIW, both here and at our club, the track power and the Legacy base are powered from the same switch, never had that kind of problem.  The base puts out the 455khz carrier immediately when powered, and the TMCC/Legacy locomotive waits a spell sensing if it has the TMCC signal.

I'd check to make sure that the terminal on the command base is going to the track or the "U" terminal on the power supply.  Then check to make sure that the three prong wall wart plug is a) still a three prong wall wart (we've had people break these off or use cheaters to get around using a three prong outlet) and b) plugged into a properly grounded outlet.  If either condition is not met there won't be a viable command signal and the loco's may take off at warp speed.  

I know that the TMCC base never had issues on power up but the Legacy base does go through a more elaborate self test.  I've seen my base go into PO'd mode once or twice where I had to shut down cold and restart.  The loco's didn't take off but I wouldn't rule out that possibility if whatever is wrong is enough to delay the track signal from coming up with the .2-.3 seconds that the engines require for poling purposes.  That's why you want to have a kill switch to shut down everything when something goes really wrong.  A Legacy base with issues blinks the color coded LED's on the bottom edge of the base.  There are combinations of colors and count blinks that should indicate what's wrong.