The arguments about not subjecting others have been made already. I appreciate that these will sometimes not resonate with people who feel they have no other choice.
Somewhere between 10% and 30% of those who commit suicide by colliding with a train survive. This is quite poor, especially as most survivors will be left with serious injuries: they might need an arm re-attaching (and it will never quite work as well again), or maybe it won't be able to be reattached, or they might be paralysed.
A bus is less deadly. It's slower and not as massive, so doesn't have as much energy. You can absolutely die from being hit by a bus, even in an urban environment. It seems likely to me that well over 30% of people survive. It also seems likely that more people survive and make a full recovery, but probably a majority are in considerable pain for considerable time, including some of those who die. You're less likely to get the quick decapitation or "mistification" occasionally associated with high-speed train suicides, and instead die from blunt-force trauma, with lots of broken bones and internal bleeding. It's less like jumping from seven storeys and more like jumping from two - very risky for someone who doesn't want to die, and not a certain or quick enough death for someone who does.
