[Jetguy]

Well, looking up your engine at Lionel parts https://www.lionelsupport.com/Repla...AB6E615D9B4ED6&CFID=27837079&CFTOKEN=94353204
the problem is, this motor has NO flywheel so it will come to a dead stop with very little coasting.
So yes, just kind of how this model works.

 

[Jetguy]

Again, sorry, you seem to have purchased what I would not consider one of Lionel's best engines.
It's not terrible or anything, but as you can see, single can motor drive with no flywheel, no provisions to add one, so it kinda runs how it runs.
Your other engine is completely different in motor truck style gearing.
I think you have this RS-3 engine 6-18814
This has was I call sidewinder style motor trucks where the motors and gearing is NOT a worm gear and thus the motor even without a flywheel is free rolling when power is removed.

 

[Jetguy]

Again, you gave the number of your SD-18 6-18806 which I looked up in Lionel parts and validated, it is single vertical can motor with a worm gear drive that is NOT what we call backdrivable where the wheels can spin the motor when coasting. Since the motor has no flywheel, then it tends to stop more quickly, and then also, the wheels and momentum of the train cannot backdrive and spin the motor.

The other engine you are saying reacts differently and coasts:
You did not give the number of your D&H RS-3, however, some googling shows only a few models were made.
The first one is also the same rough time period as the SD-18 6-18814, which unfortunately, did not come up listed in Lionel parts or diagrams. However, more searching for pictures on the web of that specific engine show the motor truck and gearing which is free rolling and backdrivable even without a flywheel.

The other one that came up is a modern Lionchief Plus Delaware & Hudson LionChief Plus RS-3 #4121
Which DOES have flywheels and thus also would coast.