Technically, you should never rely on track pins to conduct electricity. On my layout, every 3'section of GarGraves gets its own set of feeders, power and ground. It's extra work and expense to be sure, but my trains never slow down, unless I tell them to. I don't know how big your layout is, but I'm pushing power out to around 3500 feet of track.
Regarding Amps and Volts, if you were to think in terms of water and not electrons, voltage is akin to pressure, and amps are the rate of flow. The wire is a pipe. The larger the wire, the more current can flow through it.
Now consider the track pin. The rail is a fairly large pipe or wire, the pin is rather small by comparison. So if there is any dirt, or loose contact, guess what? There go your volts, like a leaky garden hose.
Short of adding feeders, there is a quick fix. Take a pair of pliers, and squeeze the rail around the pin, both sides of the joint and all three rails, and both ends of the bad section. However, feeders are still the ultimate solution.
