I really don't think that stopping to take or pose for a photo when walking along a public right of way constitutes tresspass.
According to the Railway Regulation Act, Trespass can be prosecuted where the person is refusing to leave (tenuous in this instance, there's no evidence a challenge to leave was ever given) or where someone is making a nuisance of themselves on Railway premises. I'm not a lawyer, so I'm not sure if potential for disruption or misusing an authorised crossing counts as a nuisance, but if it doesn't then I'd certainly like the law changed to make it so.
www.cps.gov.uk
Perhaps it would help if the press releases describing people hit by trains were really lurid about the injuries, and vague about when they'd have lost consciousness. I suspect people choose suicide by train partly because it sounds like a quick and certain death, so convincing them to try something else at least helps the railway. OTOH, it would hurt grieving relatives, who can currently take comfort in the idea that it is an instant death and the deceased never had time to feel a thing.
I can't imagine that's going to help anybody - for one, there's no way of confirming how much pain those who do sadly pass are in and for how long. Many who do get struck by a train are killed instantly. If a person is in a position where they are contemplating suicide the chances are that they're already in a vast amount of pain, mental or physical. Being made aware of more pain is unlikely to alter their decisions. As you say, those left behind would be even more hurt than they already are trying to come to terms with why their loved one has died.
Suicide on the railway is best addressed by focusing on helping suicidal people's mental health. Not by handing out free tickets to Eastbourne for Beachy Head. This is something the railway is actively involved in with the Samaritans, but it is an issue that needs much wider support.
Further, the tactic of being very visual about the consequences is in use for many TV spots and hasn't stopped the vast majority of misuse, which is people simply ignoring or not comprehending the dangers. To address this, probably the first stage is to review legislation to make sure it's fit for purpose, and clear that stopping on a crossing is absolutely illegal. Penalties also potentially need increasing - perhaps making parties liable for all costs incurred by NR and the TOC (these can run into 6 or 7 figures). I'd also make misuse in a vehicle 10 points on the license.