tpc on track 4

[Steve Wise <swise@aoot.com>]

The TPC on track 4 seems to only work in cmd mode which is bad if you're 
running conventional.  I tied setting it in conventional mode, but then 
it would power up.  In CMD mode, you can vary the power, but by default 
it comes up to max voltage.  And it seems to ignore conventional cmds 
like <dir> which should cut the power momentarily for conventional E units.

My dad is bringing up a replacement for testing after the run tomorrow.  
We'll see if we need a new one.    If mine works, then we'll use it for 
Sunday's run.

But for tomorrow: ONLY CMD TRAINS ON TRACK 4!

Your friendly FAT controller...