Welcome to the forum!
Trains not running smoothly can be caused by many different problems.

The wheels and roller pickups of the locomotives need to be clean and free of dirt/grease/crud/rust. The track also must be clean, and the connection pins between tracks clean and tight. Use hardware store naphtha for all your cleaning/degreasing needs. Light rust can be cleaned off with a Scotchbrite pad.

Fasttrack is known to have some problems, causing power dropouts and poor train performance.

A drop of oil on the axle bearings of the locomotives may be needed to get them running smoother.

If you are using the transformer in the bottom picture of your post, it may have just marginal power for your layout.


Larry

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