I don't think the 2 trains on one track analogy works with gas flow. A single cylinder port exhausts every two revs. At 3000 rpm, 25 exhaust pulses go screaming down the tube every second. That is one exhaust pulse every 4/100ths of a second. With the siamese tubes 50 pulses fire each second or one pulse per 2/100 ths of a second. The tubes carry twice the volume of gas per unit of time. Trains don't run that close together and they don't expand like a hot gas.
