I strongly suspect the pin-connection inbetween trackpieces being the culprit here.
When the train does run on the track, but very soon after starts to slow down to a halt, it would signal that the track itself is poorly connected, and it gets a little less "juice" every time it passes a set of pin-connectors, when moving onto the next piece of track.
If so, one solution would be to solder the entire track together in sections, and place a connector-clip from the transformer on each of those sections, to ensure every part of the track gets a proper power supply.
Even with thinner 0.75mm2 (18 gauge) wire, a 2amp load, and 30 feet of wire, the voltage drop at 15Volt would be under 1Volt, which shouldn´t interfere like this.