The way I see it the song is about living life and time in general. You cannot stop the time you're living in, it's always moving forward.

"Stop this train, I want to get off and go home again, I can't take the speed it's moving in, I know I can't, but, honestly, won't someone stop this train?"

He wants everything to stop and back to childhood where everything was good, it's about not wanting to move forward to the next step of life because you can't take everything that comes at you. You can't take the hit of responsibilities that hit you like an avalanche, for example, finishing university, getting and living with your new job (most of the time a job you don't even like at all), starting a family, losing family, friends, or your dearest ones.

You don't realise that you're getting older and you're losing things that you love and life is getting harder for you. Sometimes you feel like your world is falling apart because you can't adjust to the speed of everything that comes at you. You always hope that you can just stop the time or get into a time machine and live life when it's good and when everyone's still around, all over again. He knows that he cannot stop it but he always wishes for it hence "but, honestly, won't someone stop this train?".

Then he talks with his old man about life, and his old man said basically when you're older you'll rethink that life must go on. And he's been there before of not wanting everything to move on, but saying we can't stop the time and everything must go on. And then, in the end, he writes that realisation that we cannot stop the stages of life, and try to accept it.

I really like this song, probably my most fav of John Mayer, ever, it means a lot to me. Back when it was released, I was in my teenage years and had been listening to this on repeat. As I get older I didn't realise how this song really speaks of the reality I'm living in. Now I'm almost 30 I often wish that everything can stop and I can go back and do it all over again. I'm not yet in the state of being able to accept it and move on.