The RHTT sprays water toward the rail head at a very high pressure. The resulting spray that bounces back of the rail would be enough coat anyone who is close enough.
The weedkiller train would also be unpleasent to be on the lineside when it's working.
This is a subject I have brought up on here before, and one that never seems to go away. There seems to be a thinking within the idiots that because it was OK to wonder around the railway back in steam days (when trains ran less frequently, and often slower) that it should still be ok on our high intensity, high speed, rail network.
Carnforth is a place where it happens a lot. When a southbound train sits in the platform, the only place to get the whole of the loco in shot is down the ramp, right next to the WCML with Pendolinos screaming past at 125mph. I've even seen parents with little kids down there before now.
I have also been on the end of platform 9 at York when some people that were previously on the platform taking numbers suddenly appeared on the trackside in an orange jacket they got off ebay, taking photos of the DRS 57 in the siding.
I have seen them on numerous occasions from the Scarbrough Spa Express. The worst one was at Knaresbrough, standing in the 4-foot of the Leeds-bound line in the tunnel at the end of the platforms, with his back to the potentially oncoming trains taking a photo of our engine. Luckily (and potentially life saving for him) one of the engine crew spotted him and gave him a good shouting at.
Its crazy what these 'enthusiasts' will do just for a photograph.