Mineral Spirits are safe to use on Lionel Train motors, and last I knew, didn't cost any more than alcohol.
If alcohol is safe in most cases, then that means there are cases where it is not safe. So why use it?
Years ago I shorted out a couple of armatures using alcohol, and have not used it since.
Some folks like to use automotive products to lubricate trains, but I don't think anybody has advocated using motor oil on gears. It would be motor oil on bearings, and Lucas Oil Red N'Tacky grease on gears.
Personally, I prefer to use Labelle products: 106 grease, and whatever light, plastic compatible oil is handy. They make a few.
It sounds like your trains need a good cleaning and lubrication.