There are a lot of very good reasons why trains, buses, to include school buses do not have seat belts. Number one on my list is the high risk of spinal cord injuries, that result from just a belt. Expense, risk of law suits, making a driver responsible for someone else wearing a belt and wearing it properly, and there is more.
Even full belts with across-the-shoulder/torso? Hmmm, never heard of those other factors against belts on school buses, etc. Thanks for shedding some light.