Concerning the comments pertaining to engine speed - and I've said this before - Yes, most traditional Lionel transformers have a minimal starting voltage to the track of 6 volts. So wiring the engine's motors in series will help if you are using one of those transformers.
I highly recommend one of the Lionel 1033 transformers for running these sorts of engines. With the two settings to the track (A-U, 5-17 volts) (B-U, 0-11 volts) are very useful for either running this type or a postwar/MPC type of engine. With the B-U setting, I can make my engines crawl with no rewiring of the motors.
Also, let's be honest: These are low end starter kinds of locomotives. The Mabuchi motors used by K-Line were not always precisely the same. I've got a couple of the single motor S-2 Southern Pacific DC sets that were once made by K-Line. One of those engines operated just fine out of the box and pulled a suitable length train with no alterations. The other one needed some modifications to do the same thing.
Another observation, K-Line reissued train sets without necessarily cataloging them. There were several MP-15 train sets: A PRR, a Santa Fe and a Burlington Northern. These sets all originally came with 027 track. Later on, without being in a catalog that I know of, these sets were available again but with the new K-Line Shadow Rail track.
As an added observation, RMT used the same basic tooling for their revised/improved S-4 Bang and Buddy RDC locos. BUT these versions had a motor with different specs than the K-Line motors. Using the same B-U setting, the RMT engines will barely pull a train at full throttle.
To be certain whether it was the motor or changes to the circuit board, I stripped down one of the RMT S-4's, removing the circuit board and wiring the motors direct. I have the option of running my layout on DC current also. And sure enough, the stripped down S-4 still needed more power to run. I've also done the same test with K-Line engines and they all run fast with a typical 6 volt minimum transformer, whether they have the circuit board or not.
The one starter set engine exception is the K-Line steam engine, which runs very smoothly and slowly. The two cons of that engine are the Seuthe smoke unit and the plastic eccentric value piece, which is prone to breakage.
