You could use this, not still for sale from MTH but often on eBay.
Note that this IS NOT a DCS remote Commander. There are only 4 square buttons and a throttle. DCS version has more square buttons.
It allows the remote to control voltage from the transformer. Transformer connects to bottom of grey receiver, track connect to the tabs at the top. If you don't use realtrax, just connect the two tabs to wires going to the track. Works on infrared signal... great to control a single conventional train.
They work great! Wish they still made them.
Ed
ps. usually less than $40
This would be my vote as well, as it would run all of your trains (of course, only one at a time) conventionally.
If I remember correctly, the remote and lock on also came with a 75 or 100 watt brick and was offered in several railking r-t-r sets.
Jim