Comparatives

I would use the superlative, rather than the comparative, but your meaning is perfectly clear.

It is possible that I didn't answer the whole question. I would use the superlative for the question, but you can use the comparative for the answer, so:

"Which is faster fastest, a train, a car, or a bus?
A train is faster than a car and or a bus."

I think the rule is 2 items use faster. More than 2 items use fastest.
Which is the fastest, a train, a car or a bus?
A train is the fastest.
Which is faster, a train or a bus?
A train is faster than a bus.

As you've phrased them, all are correct. But the rule you suggest (that a sentence with more than two items requires the superlative) will break down in a sentence like "A train is faster than a car or a bus." To use the superlative, you'd have to rephrase to something like "Of a train, car, and bus, a train is fastest."

Are this acceptable? Just a comparative.
John is taller than his mother and father.
A whale is larger than a shark and a dolphin.
Thanks. Fulvio

You're correctly using comparatives in those sentences.