I have learned a great deal over a short 3+ years in this hobby. Recently- I had a couple of experiences that have caused me to realize these "rules" and confirmed maintenance and prep practices.
I favor Lionel (over MTH) generally. I don't know why- but I do, so these apply to Lionel products, but maybe to ALL. I also evolved from postwar/ conventional, to now command control and TMCC or Legacy engines. Most are bought on e-Bay.
First- ordering a BTO (I call it OaW- order and wait): I pre-ordered and, received 9 months later, a Legacy "F7" AA set. The un-powered unit had a "dead short" in the trucks and, had many loose screws. So (disappointed that a $900 set wasn't "right" out of the box, too long on the phone, worry, then wait) - I had to get an RGO from Lionel. They sent a prepaid shipping label, found a short in a truck, but also a need for a new sound board and smoke unit. They did take care of it, and in only about 10 days ship out to return.
BUT the excitement of a new BTO loco was lost.
A NOS Legacy GP7 (2008 release) After running the locomotive for about 30 days a gear "froze" It of course was not in warranty. This caused HOURS of time figuring out how to take everything apart - my first loco with "LionDrive". (Hint: I looked and looked for the screws to remove the power truck- then saw a Lionel video showing the "turn it left 90 degrees to remove the power truck" trick.) .
I found the cause issue was that the Chinese subcontractor had not oiled the gear train fully. (On the plus side- there IS a lot of learning, and a degree of satisfaction in doing this for myself, but...).
(update: when I put the loco on the rack, as soon as I power up in command- it took off uncontrollably. As I removed the shell, the flywheel fell OFF- and that is what makes the control, control. It has to sense the speed.)
So- my RULES: If one wants to have "state of the art" locomotives (vs. earlier, "Made In The US of America" Lionel), and if one favors command control and newish, TMCC and Legacy engines:
1. Realize that a manufacturer/importer is really in charge of the DESIGN, not the execution/ manufacturing of that design. So, in this case- Lionel is in charge of the features, but (???) is the one who failed to oil the gear and didn't tighten the set screws on the flywheel. The reality is that an importer in this small production market cannot control QC the same as if he MADE the product. Lionel cannot station QC people in China, even IF the subcontractor would accept it. It is not LIONEL's QC- but the subcontractor's QC that determines what you get.
2. Complication: Look at the dramatic difference (wires, connections, boards) between a PW, and more so a TMCC and a Legacy engine. The more features, the higher likelihood of something not being right even in a brand new product.
3. New Old Stock ("in the box and un-run" ) does NOT mean perfect. These are NOT automobiles and there are no recalls or re-engineering out the faults that appear "in early production". It is ALL early production. Normal "production runs" are ONE - run of maybe 1000 or so, about every 5-12 years.
A problem "common" in one particular type of engine will be "common" in all of that run's engines, maybe yours. If you buy a NOS locomotive the manufacturer will likely not have not "fixed" any inherent problems. And there is NO warranty, so...
4. Buy engines that are a few years old. This allows that a previous owner "worked out the bugs". Most of us do not really run any single engine a "lot" so a used engine in good shape is probably a good buy. But you can't tell "maintenance"- so one with a lot of hours may or may not be a good buy.
5. "Fiddling" is expensive in money by a guy paid to do it- or by YOU in time. Be bold enough to take it apart and learn to fix it yourself. Also- the chances of YOU fatally screwing up something, are remote. And if you do- see Rule #6 below.
6. "For Parts"- Expect and pay for a "For parts" product, on (say) e-bay. But check for an underlying gem. (I bought a Lionel LC+ RS-3 "for parts" at $125, and it was like brand new- just needed a re-solder of the main power wire from the collector.)
7. Test- then maintain. For or ANY new-to-you locomotive, NOS or all-new. run the engine to be sure it is electrically and electronically sound/ everything works. Then, immediately put it through your personal maintenance process- clean the wheels, oil the gear shafts, side rods and axles, lightly grease the gears, check and tighten the screws slightly ( but don't over-tighten).
8. Have a maintenance schedule: after you complete an overall maintenance, record the date. I use pencil written in the bottom, with the date "Maint 8/20", then I know when the next might be due.
Still learning....