I have a Lionel ZW transformer older version 1955-1956, And lately my train loses power at one area of the track and then regains power again. This happens on both the outside and inside tracks.

Just wanted to add a note to this problem you are having....Question is this...How big is the train layout and how many power clips do you have on the track??
Reason is being if you only have one power clip for the whole track, you will notice a decrease (slowdown) of the train in certain area(s) so i would recommend adding another power clip or two to your layout, also If you are using old regular O track, make sure you have the metal pins in all 3 railsHope this helps

Bad track connection.... clean the track with dry scotchbite pad, then wipe with soft cloth, or the connection pins are loose in the track. tighten with needle nose pliers

This does not appear to be a problem with the ZW. The XFMR does not know anything about train location on the track. It appears to be a track connection problem on the center or outside tracks or both. One way to identify a bad connection is to take a parallel wire from the common and hot terminal of the ZW and connect directly to the area of track where the train stops and slows down. If this cures the problem then I recommend checking the tightness of the three track connections. For both FasTrack and Tubular, crimp slightly the pin receiving holes in the end of the track.

Note: For both tubular and Fasrtrack it is recommended that common track power for any one section be input to the track at more than one point. In this way one bad outter or center conductor won't cause the train to slow up or stop.

If FasTrack, you can solder directly to the tabs on the track underside for a permanent connection.