A plastic boat. An electric cutter and weights.
Row out in the night. Mount the weights to your feet. Throw away the keys into the deep water.
Cut into the boat. Position yourself as it's slowly deflating.
Cut the main arteries of your neck, then let yourself glide off the boat.
Ideally drink poison before you cut into the boat, if you can get something deadly.
This way, you have one or two failovers.
I thought intensively about how to securely kill yourself and this seems the best method.
In comparison, throwing yourself in front of a train does not guarantee your death, but my method does.