Although my layout is set in the 1930's, there are occasional inconsistencies. For example, if you look in a drawer under the layout, you'll find a pair of 1950's diesel locomotives on hand. They're still SP, one in the Black Widow paint scene, and the other in the simplified "pumpkin" black and orange scheme. They're important - they're my insurance policy.
One of the big rules in operations is that a layout needs to run well, and that means the locomotives need to run well. No locomotive, no train, no operating session. Worse, an operating session is much more wearing on the trains - hours of running, multiple locomotives running at the same time, rougher operators, less respect for known trouble spots - so even engines that work perfectly for you on your own can fail during an ops session. Having a few spare locomotives on hand becomes necessary when one of the locomotives develops a hitch in its mechanism, or a coupler breaks, or a locomotive stops running completely.
My operating sessions tend to use the same locomotives - a few nice-running Consolidations - that can pull all the needed cars. Some lesser trains run during the operating session use some weaker locomotives that can be commandeered in a pinch, giving up pulling power or sound. The passenger train locomotives can also be useful in case of problems, but my two 4-6-0's tend to derail going backwards which makes them poor choices.
And if all goes completely wrong, then the two diesels get pulled out, and finish the operating session. The show must go on, and the trains must complete so the crew has fun.
Luckily, the diesels hardly ever get pulled out.
Unfortunately, they were needed when I had some neighbors over earlier this week; the layout hadn't run for months and I'd just started on cleanup when they stopped by.
- Engine one stopped moving as the motor whined. (Pulley started spinning on axle in a painful-to-disassemble Bachmann Spectrum locomotive, annoying to fix.)
- Second engine was stalling on the dirty track.
- Third had a bind in the mechanism.
- Another had electrical contact problems.
Each one got pulled aside and it's replacement took over the train. Finally, no more spare steam locomotives were available; the diesels go pulled out and saved the day.
That's why I'm spending the next couple days on tuning locomotives, including tearing apart the very awkward locomotive to get to the slipping gear. It's time to fix those mechanisms, touch-up paint, and get some new locomotives ready for the layout.
Moral: always have spare locomotives if you're planning on doing operations. And don't forget to hide a pair of diesels in a convenient drawer.