So you finally got every nail in the track and throw the switch, and nothing happens. Unfortunately, a number of things can go wrong with a model locomotive, and not all of them have to do with the engine itself. For the most part, poor performance will come down to either electrical issues or cleaning and maintenance issues, either in the locomotive itself or on the track. In any case, a process of elimination will generally indicate where the problem lies.
Build a Test Track
Tack down at least a foot of spare rail track on a plank or an unused section of the layout board. Cork roadbed is optional since this is only for test purposes.
Attach a pair of lead wires to the track and then to the power supply. The ends of the track can be open.
Place the locomotive on the test track and slowly increase the power. If the lights come on and the engine begins to move, that means your problem is somewhere in the electrical connections on the main track, not the locomotive.
Inspecting the Engine
Dislodge or unscrew the body of the engine from the wheel base to reveal the interior motor using micro-screwdrivers.
Shine a flashlight on the motor carriage, looking for debris or loose wire connections including degraded solder. You may have to resolder broken or stressed lines.
Brush out any dust or debris with a small, flat brush.
Dip a corner of the cotton cloth into the rubbing alcohol and gently pull the cloth over exposed areas including the wheels to remove dirt.
Retest the engine on the test track.