The standard loco used for passenger service is a Bachmann F7 Pancake Motor model, so called due to its "pancake" motor, having a flat 3-pole round rotor. I have standardized on this model, due to its common availability (can be had for $10-$12 used on eBay), and ease of service. This is a trainset quality loco, but with some tweaking we make reliable runners out of them. The pancake motor itself is never the issue, it is a nice little motor. The problem with it is getting the power to it reliably. (Some Like-Like locos use the same motor).
The plan is to run the locos in mated pairs. First we will make each loco operate reliably independently, then wire them together. This results in a two-motor loco with double the number of electrical pickup axles. This arrangement makes it immune to any dropouts of power, since the pickup axles are now spread over a greater distance. Now it is a strong puller and will not stall over turnouts.
1. Find appropriate locos. We search eBay for "HO Bachmann F7" or "HO Santa Fe 307" for possibilities. Look at photos closely. You can generally identify a pancake motor model by the odd notch in the fuel tank area - see pencil point in photo. Every one we have seen with this notch has been a pancake motor.
Electrical tuneup: Remove the sideframe housing from the pickup truck. Remove the wheelsets for cleaning. Clean the treads and the back sides of the wheels as well (I use a little wire brush in a Dremel tool). Brighten up the wiper strips as well. Place a tiny dot of oil at the four points at which the axles rotate within the sideframe (this should cure any locos which require a little push to start).
Drivetrain tuneup: Remove the sideframe housing from the drive truck. Place one dot of oil on each gear. Install new traction tires. I like the ones from eBay seller "ggnlars".
Lighting upgrade: Remove the stock lamp and replace with a 12v LED (All Electronics PN LED-12W) and a 1K resistor (All Electronics PN 291-1K) in series. Be sure polarity is correct - it will only light in one direction of travel. This will provide directional, constant-intensity lighting. The LED will be at full brilliance before the loco has enough voltage to begin moving.
Weight check: I add weight to 12 ounces - but that is a personal preference.
Test each loco independently. Make sure it starts without nudging, and runs smoothly. If it requires a "nudge" to start, that indicates friction somewhere. See the note in Electrical Tuneup about lubing the axles.
Once they are operating 100%, wire the locos together. I run paired #24 speaker wire between the locos, passing it through the rear window of each unit.
After shells are replaced, form the wire into a U-shape to allow flexibility.
I leave the stock horn-hook couplers as is to join the locos.
Where else can get you a dual-motor loco for the price?