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I recently got a used bachmann HO scale dda40x, it works but at first it stutters a bit and jerks around. Sometimes it just stops; it goes around the corners without getting stuck, it just jerks around. However after about 30 or so minutes it smooths out a bit. I have cleaned the track so it’s something wrong with the locomotive. It’s jerkiness ruins the experience.
 

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Chief cook & bottle washer
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I recently got a used bachmann HO scale dda40x, it works but at first it stutters a bit and jerks around. Sometimes it just stops; it goes around the corners without getting stuck, it just jerks around. However after about 30 or so minutes it smooths out a bit. I have cleaned the track so it’s something wrong with the locomotive. It’s jerkiness ruins the experience.
You don't say if it's a DC loco or DCC.
Make sure the wheels are clean as well as the track. If this doesn't improve things, it's time to start taking it apart. First thing to check is that the axle bearings are in good condition as the plating can wear off on the chassis. If it's OK there, strip it down further to check that the grease hasn't solidified on the gears/worm wheel as this can cause problems too. It's worth doing a proper job on a second hand loco to get the best out of it.
 

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In depth idiot
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I agree with the above, and would add check the electrical circuit.

Bachmann use this mechanism construction in some models for the UK market, and the technique of using wiper contacts in the electrical circuit would top my list of reasons for the symptoms described.

First quick test if this is a DC loco: on the bench, connect the motor terminals directly to your DC controller output. If both motors run their drive lines smoothly and reliably like this, then you may be sure that there is no drive line problem, and can concentrate on finding where interruption of supply occurs in the electrical circuit.

Straight up, I have all my Bachmann bogie traction models apart immediately after purchase, test motor as described (never yet a problem!), and then solder in fine wire linkages wherever there are wiper contacts in the circuit. And they run reliably,,,
 
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