Some years of 0-27 used hollow pins. When these get old they rust from the inside out. They have a small seam and can be crushed by pliers. Remove these from center rails and use solid pins there. (bad hollow can get red hot if thin enough. The outer rails matter too, just less so. Outers combined have twice as much pin metal as the center rail has. You need more metal to flow more amps.
You may have an engine or car wire(s) shorting. Check the tender roller wire at the terminal, the hold down tab(if) and chassis wire hole. Etc. etc.
The cleaning of the armature gaps and brushes can improve low end performance, lower the voltage needed to move, run cooler, and prevent some stalling.
Really dirty wheels may need to be scraped too. Sometimes the grime appears to be steel but isn't. Try a small screwdriver blade. If you see silver metal to metal scratches(light) you are maybe clean enough for alchohol or naptha on a rag or Q-tips. Thick gunk takes forever with solvents alone sometimes. I test scape, then decide.
Add a track feed/drop to each leg of a turnout so power can travel around a switch, rather that thru them. This is in a lot of old lionel instructions as the "better way". As the tab contactacts within the switch age they may not pass the amps they once could, so it helps tons. So will more power drops in general. (if you get the chance, go with new track. It will make a huge difference in running . Look at Menards O prices ( O is thicker, stronger, taller than O-27, and then your looking at new switches too, but the O ones are better than 0-27. I mostly run 0-27 myself. )
I like to add power in the curves or near them so the loco slows less there, evening the overall speed. (lock ons don't like curves. I solder wire on. Way better than a lock on too; one soldered connection vs two clips (wire/trck), the solder usually has less resistance 
14g wire should be fine really if the runs aren't real long(chart). Stranded would be better but not by much. Speaker wire tends to have a softer less protective insulation, but the AWG rating is the bottom line... read the wire. It's specs are often printed on it if you look REAL close . (tons of charts and free calculators are a search engine away... X amps at X volt at x ft long = mininum wire gauge)(voltage isn't usually an issue, most are rated far above our low volt train stuff). Bigger wire is better for power delivery. I wire to handle my transformers max output. Smaller wire than that needs a fuse to prevent frying wire during heavy shorts. Check your 14g against all that.
A fat wire will handle a direct short and the internal breaker becomes your fuse. (extra inline fuse/breaker for train's max amp draw rating is for protecting trains and or accessory drops (always fuse those where thin acc wire ties in to a heavier wire.)
Another old track issue can be bad, cracked, shifted insulation under the center rails. This can be hard to trace as the short may only exist sometimes, usually as an engine is on a track. Heat can be felt there usually. (careful of blisters ) Just pry up tabs, pull the paper out and replace it. You can use cereal box, matchbook covers, notepad backing, etc. (same for making your own isoalted outer rails to automatically trigger some accessories, or extend the anti-derail feature further, beyond a turnout. [isolate the new rail and move the plastic pin to it instead; simple])
Other issues can arise from roller spacing and shoes shorting in the turns. Some trains just don't do well on some turnouts. I have two small pieces of tape in spots on my switches to help two engines that navigate the other four turnouts just fine. (and one is a 2046 Hudson, )
Finally, some diesel rollers can fall off of 0-27 center rails in curves. I have two like this and simply limit how far the arms can drop with a tiny zip strip. They would drop low, arm axle boss's tab shorting on the tie's tabs and slide back on the rail at rest, but now in neutral.....or reversed away from it. They also jammed the loco to a complete stop sometimes.
This gets much easier once you read it and live it a while. "Old hat". It will become too easy and pass without notice
