Sorry to hear you've been through that, I hope you're in a better frame of mind now. Needless to say,
if you haven't already spoken to the Samaritans, about your incident or anything on your mind, I'd highly suggest it. Most people, even the driver (although perhaps not in the heat of the moment!) will understand that to do something like that you're not acting yourself or thinking rationally - you're not a monster for having done it.
Regarding your railway-related questions:
Depending on the train, drivers can't see directly in front of the cab (simmilar to how you can't usually see the bonnet of a car when inside it). This "blind-spot" might be a couple of metres in front of the train or it could be almost ten in some cases. Thus, it's entirely possible that even if you jumped at the last moment, you had already disappeared from the drivers view and were 'under' the train from his/her perspective. However, the lack of an impact noise and the subsequent investigation would likely establish that there was no fataility or impact. Regardless of that, though, there's no hiding the fact it would have been very upsetting for the driver at the time.
Some trains have forward-facing cameras. Almost all new ones (last 10 years or so) come with them and a number of older units have had them retrofitted too.
The Police would have attended as, at the very least, there's been a tresspass incident, but potentially also a fatality or severe injury. They're there to firstly establish if there's been any crime committed (someone being pushed in front of a train etc.) and following that assist in taking witness statements and handling the decesased (if applicable). In your case, I imagine their biggest concern was for your welfare and finding you lest you had received a glancing blow/needed immediate medical attention - they're not there to go about arresting people in those situations.
If you're concerned about getting into trouble with the police regarding the incident, I think that's pretty unlikely. I can't imagine trying to prosecute someone for trespass in such a vulnerable state of mind would be at all productive. There's a big focus at the moment to reduce rail-related suicides and thus treating survivors of such experiences as criminals wouldn't help at all in this regard.
As mentioned, by far your best option at the moment is to contact the samaritans about the incident & your feelings towards it - they'll be able to offer much better general advice going forward than any of us on here (even on some railway specific questions, as they've got good links with the industry). Perhaps they might even be able to anonymously pass a letter of apology & concern to the staff involved in the incident, if you think that would help you - it's worth asking.
