Johnross and Martin give good advice. Replace the transformer with a new or rebuilt one through a dealer or model RR club contacts. The rest is mostly elbow grease and time, as noted by johnross. Ebay also is a good suggestion. More track and rolling stock for my American Flyer set-up, as I remember from childhood, were a constant demand of mine. I still have a vial of whale oil (don't know how it got kept all these years), but only some HO trains packed away.
I got inundated with mail order catalogues this year, and several had starter Lionel trains on offer. Are they coming back?

Don't get hooked! Some poor souls are seriously afflicted.