The device in the tender is the reversing unit. The most common problems with it are the mechanism that rotates the drum gets dirty and sticks, or the contact fingers that ride on the drum wear out. Worst case is the operating coil is burned out and the entire unit will have to be replaced.
There should be a direction locking lever sticking out of the bottom of the tender. You can use it to manually operate the drum and see if the operating pawl is sticking. If it is, usually a shot of contact cleaner will free it up. If it works manually, but not with power connected to the tender pick-ups, check that none of the wiring has broken loose. I've seen some where the wire was broken, but the cloth insulation was holding it in place so it looked okay until you touched it.
If you need parts, the numbers are:
XA9612CRP - Top Fingers XA9612BRP - Bottom Fingers XA8716 - Drum
Parts can be obtained from Train Repair Parts
or RFG Co.
. Both have exploded view diagrams and wiring diagrams, but the TRP diagrams are easier to navigate. The diagram for you loco can be found at:
If your loco has the four-pin jack panel, you can test the motor directly by connecting the the lower jack to the one above it on each side of the jack panel with a jumper wire. A heavy paper clip will do in a pinch. Then connect a power lead to each jumper. Like this:
+-------------------------+ | o o | | / -Jumper- /
| | o o |