I think people see it as a quick and easy way out of their problems, especially during these tight economic times. They don't tend to think about the consequences of their actions, especially to the poor train driver, the people who have to clear up "the mess", or even their own family. Quite selfish really.
As I've already said, people who're going to kill themselves in such a spectacular fashion rarely plan it like this. They are clearly in need of psychiatric help for the most part, and they've either not had the help they need or the help has not proven worthwhile. Sadly, this is often the case with mental illness by the very nature of it.
People who plan to commit suicide (and I've heard of a few) usually go for more sensible approaches like ODs, poison and hanging. It may be selfish, but again the people really are severely depressed and cannot/will not seek help. As I've already mentioned, this is the case of mental ill health.
Most people who jump in front of trains for the purpose of suicide have, in my lay opinion, probably not been planning this for some time. Probably people who have suffered a major emotional upset, such as just losing their job or a loved one, going through severe financial stress (bankruptcies and so forth), divorce proceedings...
Let's be fair here; if someone jumps in front of a train with the intent of killing themselves, it is pretty fair to assume that they're not of sound mind. As I've said above, people who plan their own deaths will generally do so with more dignity than this, and causing minimal distress to those around them. Someone who's felt the need to jump in front of a moving passenger train has probably not planned the event more than a few hours in advance, is probably reeling from emotional trauma and is quite certainly not of sound mind to think about what's happening.
I find this constantly on railway forums. The jumper is assumed to be selfish and doesn't care about the driver and the subsequent disruption. However, look at their mental state. You and I would not be jumping in front of moving passenger trains; it's a crazy thought. They are not in a fit mental state to judge all the things we're talking about here when they do this. How can you expect someone who's obviously frustrated enough to do this act, which is clearly a rash act, and still think about the consequences of their actions once they intend to be dead.
I'm not saying it's not a traumatising experience for drivers. I cannot begin to imagine what it's like to be in their position. It must be horrendous. But I also cannot imagine what it's like to be depressed to the point that you feel you can no longer continue with your life. I would urge you to bear in mind that the people jumping in front of the trains have had problems, and will have problems at the time of the act, that we as mere onlookers cannot even begin to comprehend.