If the cars were deep enough in the train, there may be no way for the engineer to notice unless they derail to the point that the air hoses disengage, causing an emergency application.

Yes, the cars being on the ground would cause some additional strain on the engines, but, if it's only two cars or even only one truck each on two cars, that difference would not be much in a heavy train pulled by multiple units.

The basic rule with ties is that under normal use, concrete ties last longer and can work better for distributing heavy loads, but wood ties usually hold up better in non-catastrophic derailments. A derailed wheel may slightly gouge wood ties that are otherwise in good shape, but they can continue to be used. Concrete ties would tend to sustain cracks in these situations and would then have to be replaced. (The damage may also depend on whether the derailed cars were heavily loaded or were empty.)

The same is true for off-rail equipment. A skilled bulldozer operator can climb over track with wood ties with little or no damage. The same is not suggested for track with concrete ties.

Finally, it is not uheard of for a few wheels to derail far back in a train and to later rerail themselves at a switch or grade crossing. There have been examples of railroads finding gouged wood ties that gave a clear indication of a derailment -- but the train arrived fine at its destination and, when inspected, had all wheels on the track. Of course, the railroads do try to find where the actual derailment took place and what could have caused it.

(I'm far from an expert in this, but this is what I've read and been told by railroad employees and officials.)

-- Ernest