After cleaning the track and checking signal level, if it's anything lower than 10s, you might try a few things.
Remove any Lionel (TMCC and/or Legacy) equipped engines and powered cars from the track. In fact, remove all other brands and equipment. Like cabooses and pass cars. Just test one engine for signal, at a time alone on the track.
If you have anything drawing power from the same source as the track, remove it.
If you have a car with lights push it around the track and watch to see if it dims as it goes round.
Try connecting the outside rails together.
Make sure all track joints are solid. If any problem areas you may have to solder them together.
What brand and model of track? What power supply? how big is the layout? How many power connections?
What model TIU? any lights connected for signal?
