Welcome to the forum!
It looks like you are using Lionel FasTrack sectional track. There may be problems with some of the sections of track. With your voltmeter, start at the terminal section and measure voltage at both ends of that track. Now work your way around the layout checking the voltage at both ends of every track section. When you get to a section that has voltage at one end and not the other, that section is bad. Take it out and replace it with another good section. Do this on all track sections until you have gone around the layout. You may have more than one bad section of track.

On each bad section, turn the track upside down and follow the rails and wiring from one end to the other, checking continuity of all 3 rails with an ohmmeter. You should be able to find and repair any bad sections. If not, or the track is new, return it to place of purchase for an exchange.

Larry