I think we all understand that we're helping label... but specifically, why so many trains, planes, trucks, bicycles? I don't think it is really about training

I think we all understand that we're helping label... but specifically, why so many trains, planes, trucks, bicycles? I don't think it is really about training for self-driving AI since although these things all seen transportation-related, in many cases a lot of the images would not be relevant to a car and certainly not as relevant as other things we could be helping labeling for that effort. How much train/plane/bike/truck labeling do they need? It seems like these have be standard for several years now, which is what I think the OP is really asking. Why these images, and why for so long?