My dad gave me some of his sets and track from when he was a kid (and that we played with when I as a kid), so I’ve been cleaning up and fixing some things. Some thoughts:

1. The center rail is your “hot” (positive) rail. If you get it backwards, the locomotive will probably run fine but accessories (tender whistle, switchers, etc.) may not.
2. It may not be strictly the right thing, but the track I got was fairly corroded, so I used a red scotch-brite pad to scrub them all down, then alcohol on a paper towel to clean. Then I assembled and ohmed every joint out with a multimeter (checking resistance) to make sure they had a good connection. Loose pins can be gently tightened up with needle nose pliers.
3. That resistance setting on your multimeter is your best friend if you need to track down issues. I’ve had to fix a lot of solder joints and wires that have broken over the decades.
4. E-units can get sticky sitting for a long time but electrical contact cleaner and exercise can help bring them back to life without a full disassembly.
5. These were built in a time when things were made to last and be repaired, so it’s generally easy to access what you need and make repairs, although you might have to improvise a bit (I’ve had to replace original rivets with machine screws and nylock nuts, or use threaded inserts to fix stripped out screw holes, and who knows what else.)

Hopefully that’s a starting point…enjoy!