Comment your test track

The test track is where new cars on the layout go first. They are upgraded to metal wheels, couplers checked for height and reliability, and weighed. Cars that don't meet the standards are left on the test track until repaired.

While this produces a fleet of good running cars for the layout, there's often several days or even a month or two between diagnosis of a problem and time to repair it. If only I had some way to remember what the problem was...

A time machine would certainly solve the problem, but I'd have to meet myself to ask what the original problem was. After meeting myself and asking me what the problem was, the past me might just fix the problem to avoid forgetting what it was in the first place. (Maybe I'll get lucky and need a part.) Fine, right? But if I fix the problem I won't need to travel back in time in the first place, so I won't fix the problem.

An index card with a description of the trouble (and maybe the results of a few common checks, like replacing Proto2000 gear axles) might just resolve the above paradox and solve the original problem of not knowing what was wrong. If the index card was stored with the car or placed nearby, the information I need would be there when I needed it.

Just like in programming, leaving yourself a note close to where you'll be working later can save you lots of time and aggravation. Plus, you won't have to mess with speeds above Warp 10, flux capacitors, or repair paradoxes.