What spare parts would you keep on hand?

Which Lionel trains? The parts I keep on hand to work on trains made up through 1993 are not likely to be the same as what you would want on hand for newer trains.

For the stuff I like to repair, I keep basic consumable items: brushes, brush springs, e-unit renewal parts, traction tires, light bulbs, and a wide assortment of screws.
Also, whenever I needed a part, I evaluated whether I was likely to ever need that part again. If so, and the part wasn't too pricey, I always would purchase one or two extra.

Thanks for the feedback.  I guess I should have provided the answer to your question in my original post.  I am pretty sure most of my stuff will be newer and Legacy equiped.  I am thinking that my stuff will probably be 2010 or newer.  My new trains I currently have on order are two NS heritage SD70ACes and a PE Legacy.  I also have a PE lionchief from last year.  I also have command control switches and manual switches.  I suspect that I will be shipping my ZW-L back if something goes wrong with that.

I'm not sure there's anything you should keep on hand for modern engines. Just get things as you need them. One thing for rolling stock would be coupler knuckles and rivets.

Smoke unit wicks, if you are going to be using the smoke a lot. 

all of em!!!

I fix train myself just to keep them up and running at shows.

postwar and prewar trains need brushes and e-unit parts.

I also have a extensive supply of other parts as I have people

bring me trains at the shows to fix, that way I can fix trains while they

shop the show. Keeps em running and happy.

For yourself I would keep on hand smoke wicks as Southwest Hiawatha mentions, also

traction tires and oil and lube. keep em greased and they will run a long time.

I forgot to mention that I have an assortment of the specialty wire that is made for trains. I buy it by the spool.

I keep wire, stuff like that but nothing else.    I keep several big boxes of trains that die or I cannibalize for one reason or another, and i recommend that over time you keep whatever you might otherwise throw away.  So now, after about eight years in O gauge, I have three or four different smoke units, PS2 and 3 boards I think are good, about 5 motors, all good, trucks, wheels, etc., should I need them. 

I particular, what I have needed are:

All companies:  couplers - they see to go bad, at least when you own a bunch there is always one or two bad actors.  Both normal and electro-

Lionel: magnets that activate the chuff and the wireless drawbar sending and receiving unit.

MTH: the tether on both older (thick cable) and modern (circuit board like popsicle stick is actually sort of fragile).

Williams: I've had two motors burn out on a diesel I (same loco, months apart though)  - fortunately they were the easiest thing to find used since motors never burn out (except these, for some bizarre reason).  Now I try to keep a full stock of all motors I might need by so far no more . . . 

If/when I need a part to fix something, I always order two or three. My philosophy is that if something broke, it will break again on a similar loco or car. If you order extras of what you need, eventually you will have a decent stock of all the pieces you need for common repairs.

My experience has been, with the more modern equipment, is to stock traction tires first.  Being a member of a modular group, most equipment issues we experienced had been due to traction tire failure.  Next I would stock light bulbs for bulb equipped engines and smoke unit wicks and heaters. Keeping a couple of electro couplers on hand might be prudent along with knuckle springs and rivets.  I would also keep a metric screw assortment handy.  Other than that, there is not much else to keep on hand as most other modern engine components are locomotive specific.