YES, after a brief test run, all of my new locos get lubed with red n tacky in the gearbox. IMO, now they'll likely never need to be regreased.
Its imperative to lube the internal worn gear assembly on new diesels. I've found these to be the biggest offender when it comes to "lack of lube".
Its fairly easy to lube modern Lionel diesel truck internal gears, remove the pickup assemble and the the four small screws on the geabox cover to expose the worm assembly.
I use a syringe that was an extra baby medicine dispenser from when my kids were little. It has ml markings on the side. I believe MTH's recommendation of .5-1.0 ml of grease for gearboxes is "on the mark" for ample grease without overfilling.
This is helpful with steamers where you can't tell how much your adding through the grease screw hole.
External gears, bushings, and steam locomotive and drive rods get lubed new and then occasionally whenever needed after that. I used labelle 102 for these areas, its a thicker "light oil" and greatly extends the time in between re-oilings.
I don't run my trains that often with other obligations like work family etc, but at most, reoiling gets done once annually.
Overlubeing just tends to make a big mess on track, wheels etc, as it oozes out of the axle shafts and onto the back of the wheels.
Underlubing is probably hard to detect from an operational standpoint unless the locomotive is making a excessive gear noise and/or running rough especially at slow speeds.
Often times folks don't realize it until the gears are destroyed and the loco stops moving. I believe this is rare, but there have been posts of this happening especially on locos run for extended periods of time such as widow displays and museum layouts.
Usually, its noticing the improvement in a loco thats underlubed, after its lubed. That shows it was needed.