same loco, different speeds?

I own three Atlas SD35's, and each runs at a different speed. The difference between the fastest and slowest both from the same run is remarkable. Is this normal? By way of comparison, I have a pair of LL PA2's that run at about exactly the same speed. Since I assume I will go to DCC sometime, I guess it doesn't matter to much, but in my current DC phase it is annoying.

 

A sometimes common occurence that becomes a big problem trying to lash up a 4 unit set of Fs or other locos. Some manufacturers seem to have less issues while others the running inconsistancies become a real problem when trying to MU units. LL seems to be one of the few that will have multiple units run compatible out of the box a lot of the time.

Factors that cause this can be many, some simple others not. Lack of lube on one unit can affect the speed due to drag. Same can be said for the pickup systems in use causing excess drag on one unit when not on the other due to pressure on the wheel sets. Differences in motor windings can also affect matching speeds on the same locos.
Then there also can be problem with the electrical connections not making proper contact causing the slower unit to recieve less voltage at the motor than the faster at the same throttle settings. Also check wheel gauge. Wheels gauged slightly too wide can cause some drag, not to mention possible derailment problems sometimes.

And also you may want to check the speed of the slower unit in reverse against the faster unit. I've had differences in speed between forward and reverse before with some locos being faster in reverse. With my Fs and other units, that slight difference was equal to the others in forward in terms of speed, and that unit was permanently assigned as the trailing unit whether F or SD unit running in reverse.

Lastly the speed may improve with more break in time.