If you are talking about DC powered trains, for a normal oval or out and back layout, just add additional power feeds along the layout wherever needed.

If you are talking about some kind of track configuration such as a figure '8' loop or a reversing loop, where the track crosses itself, then there is an electrical problem. Where the track meets at, say, a 90 degree crossing, the DC polarity needs to be reversed. An auto reversing circuit is used, http://amhobby.com/products/tech/dccspecialties/powershield_x.html or a manually operated DPDT switch is needed.

Here is the product instructions for the auto reversing circuit above, with diagrams to show usage and wiring.
http://dccspecialties.com/products/pdf/man_psxar.pdf

Larry