I don't see how training wheels would help learning at all? Just wondering if there is an age when kids can not yet learn riding a bicycle, and then training wheels would enable them to use a bicycle anyway (like you wouldn't expect a 6 month old who can barely walk to be able to learn cycling, I suppose). That would be the only reason I can see for using training wheels.
Training wheels don't have to be in constant contact with the ground. With my kids I installed the training wheels several inches off the ground. The kids had to balance to use the bike effectively, but the training wheels would catch them when they leaned too far.
