I think people tend to discard trains for morale reasons rather than them not being effective. But for argument's sake, a decently fast train should obliterate anyone jumping in front of it and likewise will decapitate anyone lying under it, even at slow speeds. The only exceptions I can imagine here being down to the individual situation, i.e. the type of train/pilot, getting caught by the life-guards at really low speeds and obviously the human factor. Overall though, a squishy person should be no match for the momentum of a train.
But let's not derail (

) this thread, because this thread's about guillotines and I absolutely love the idea. I actually feared people would laught at this, but I too have thought about it and here's why I'm so fond of it:
Exactly, with this method there's simply no resuscitation, no brain damage or ward to be afraid of. This is huge, because it scares most of us immensely. Also, you can indeed die in your sleep, likewise very comforting to many of us. Personally though, I actually prefer the fact that it can be instantaneous, too, if one so chooses. This means no delay for one's ctb method to work, nor to be found and no contemplation either. It's probably smarter to contemplate one's ctb before going through with it anyway, not once your only means of getting out of it is by calling emergency services with all the dire consequences attached to it.
For a long time beheading was a means of execution for people of higher stature or seen as an act of leniency, so it can't be that bad. Nevertheless, I too have heard that one's head is still alive after decapitation, even if only for a short while, but I'd imagine this to be similar to suspension hanging. One should pass out within 15 seconds at the most. But it might very well be extremely weird and more painful due to massive damage to the neck, but again, a fairly short pain.
Overall I don't think it should be too hard to build a guillotine, because it's basically pre-industrial tech to do industrial killing. One can also test it thoroughly and risk-free until one is satisfied with the results. In fact, in this regard it pretty much tops most other methods.
The only real problem I see is the blade. Like most of us I have no metalworking experience and wouldn't know where to start. After all, it's not like you can order these things online. Although I suppose one could have a custom door or window panel/grating made, cut down an old stove etc. Really though I'm a brute, because the best thing I've come up with is two meat cleavers opposing each other with a counterweight. Bottom line though, unless I had 100% faith in my contraption I'd go head up, at least this way I'll sever the throat and won't be a quadriplegic.
But I'm rambling and for the present it's just a mind exercise anyway. I don't live alone and I'm in limbo concerning my mum.
Thanks for reading.