I took mine apart (the Trials of Hercules) a few years ago because the drive train had failed. After almost no use, BTW. Got new parts from Lionel. Not free. Reassembled (enough to test); failed within a few feet - as in the gears - again.
Threw - and I mean actually threw, several feet, hearing it land with a satisfying ka-whop! - the whole awful thing in a cardboard box. One day, if ever, it will emerge as a dummy.
The best fix for a Lionel S-class Alco switcher is to buy the MTH version. I did.
(BTW - all that took place a few months after I had had to re-attach the truck side frames with epoxy because the attachments were so thin that normal handling broke 2 of them; I repaired them and reinforced the other 2. It is a terrible little thing.)