Every time I enter a tunnel, I get to a new surface which is nearer to inferno. How can I get back to the very first surface? Thanks.

Go back the way you came down from. At the moment you can not build a tunnel upwards. But you can build multiple tunnel entrances in one surface, go down one with a train and connect them at the lower surface. After that you can take a train back through them to the surface above.
Building an upward tunnel entrance as well as entering a tunnel without a train are on the todo :(

OK, thanks, I figured it out.
I remember in an earlier version, there is only one underground layer, but now it seems to be infinite.
We already designed for the old version, and 1 underground layer is definitely enough. I don't think it's necessary to scale it to infinite. People are already planning to do underground world with this mod. The new mechanism changes this game too much imo.
And btw, I think a scenario based on railway, deserted world, looking for treasures would be fun. Trains with their inner space just like the mtn fortress and chronotrain. And inside the train, the game could be like singistics mod.