You don't mention if you have grades or if the layout is flat. If it is flat, you have an electrical problem with that section of track. You may need to solder in some extra drop wires to a main buss to get the power to that section of track. If you are trying to power your track through railjoiners, that is your problem. Rail joiners are for mechanical connection of your track only. Every piece of track on any layout should have it's own soldered drop wire to a large buss wire under the layout for the most reliable operation. If you have grades, model trains will act like the real thing and slow down going up unless you add power to them and then speed up coming down.