Re: Buses and trains - effects on balance
It probably is only of limited use in training the balance for Aikido or Judo, in the same category as wobble boards, balance balls, and the like. While these methods may be good exercise in a general sense, the skill of balancing with the feet planted for a prolonged period is fundamentally different from the way you manage balance when throwing or trying not to be thrown. When you are trying to keep your balance in a martial art, there is nothing magic about keeping your feet planted in the same place, and you pay no special attention to forcing yourself to keep them there. If you need to, you simply take a step. In fact, how, when, and where you choose to step is an integral part of the balancing skill.
Practicing extensively on a wobbly surface with the artificial restriction of not being able to move the feet might even detract from your real-world balancing skills if you did it so much that you began to confuse the two situations. You could find yourself defaulting to not moving the feet in the latter, which almost certainly is a worse method of keeping your balance than the former, no matter how good you are at it. Unless you need to be able to fight while surfing or skateboarding, use such training methods sparingly.