When is a Loco too "used"

Considering that some Lionel locos have been running just fine through 3-4 generations of family members with just routine maintenance, I would say no, there is no point at which an engine is considered "worn out". Even with newer engines, regardless of manufacturer, there are plenty of replacement parts and work-arounds to keep them running. The only exception might be if the engine receives some type of catastrophic damage such as being dropped on the floor or breaking a flange on a driving wheel that can't be replaced.

The drawback to used stuff is ultimately your "taking the sellers word" for just how "used" an item is. I don't know if you could tell  a locomotive with 10 hrs on it from a locomotive with 100. Having said that,  Mth locos record scale miles and hours of use which can be pulled up under DCS. I'm not sure if conventionally operated DCS locos still keep track, I'm sure someone will chime in.

When the wheels are about to fall off and it's current draw is considerably higher than when the locomotive was new.

I have a MTH Locosounds Baby PRR Turbine that we run for hours on end at our club displays. I do need to smooth the wire edges off of the flanges so it won't cut your fingers when you pick it up. Other than that, a little oil/grease and she's ready to go. Conservatively, I'd estimate that this engine has several hundred hours on it.

At the "broken in" phase, the amps seem to go down. Eventually we may see a rise. But not yet.

Gilly

If it looks like this...

I would say look at teh wheels and if 3-rail the rollers.   Evaluate the apparent wear.   If the flanges have become very knife-edge and/or there are grooves in the tires, it is very used.    If the rollers are grooved, it is probably very used.   On a lot of older stuff, the gears are visible, if they are worn, cupped, or chipped, it is probably very used.    You have to decide how much risk to take on locomotives like that if you really want them.    At a show, you can't take one apart and check out the gear box, but if there is extensiver wheel wear, there is most likely extensive gearbox wear.

As a rule of thumb, I avoid locos that have been used on commercial layouts or owned by clubs that do a lot open houses.   These often accumulate many more hours than their age might indicate and are often worn out.   There was a well known commercial layout that closed down some years ago.   The locos started showing up at shows, and quite a few of the then old-timers warned me to stay away from them because they had been run into the ground.

Wouldn't it depend on the price and how badly you want it?  About 10 years ago I bought a 1940 model Lionel loco having deeply grooved wheels from a yard sale.  The gears are all visible.  I just had the wheels replaced and it's been fine.

A better question might be to ask if it is beyond what you are willing to pay to have it repaired if you cannot do the needed repairs yourself. Just about anything with few exceptions, can be fixed if you are willing to throw enough money at it. There are some things I would say that make it better to pass on an item.

Rob

Too used.

The engine keeps track, not the DCS system (which just displays the info), so the clock and odometer are ticking whenever it's powered up, whether on a DCS layout or not.  (And it's not really fool proof, there are errors that can occur to make the numbers way off on an engine that otherwise functions normally)

-Dave

If you oil all the bearings and then it starts squealing again after a few minutes of operation because the bearings are so loose, then it's worn out. Depending on the work involved to repair it, it may be due for retirement.

Another fine example of why MTH is ruining the hobby.  Planned obsolescence through electronics gobbly gook is one thing, but odometers for goodness sake?!  The loco on my track right now is a vintage 1615...  lubed it up last week and let her go.  Don't ask me how many hours it has on it.  I DON'T CARE!  

I have a MTH Dealer Appreciation Big Boy from ages ago.    It was run as a display 10 hours a day for many many years.    I bought it and upgraded it to PS/2 and rebuilt the smoke unit.   I've run it for many many years.   I've run it a lot as it is one of my favorite engines and a fantastic runner.    

I can't imagine just how long you have to run a loco before it is "worn out".   Take good care of them.   Clean and oil and just take care of them and they will last a life time or more.

I own a #38 & #1912 Lionel engines. I'm the second owner. I replaced the wheels fifteen years ago. I check the brushes and keep them serviced. They should be good for another hundred years. Most all of my collection is working on their second hundred years. It just a matter of the care the units have received.