These are built in a similar to the colossal wooden coasters on this page. They are pretty much built on a hill and then I make them drop into an underground tunnel. I use the full size, large monorail trains.

Download the World's [once] Fastest Monorail here, which tops out at 67 miles per hour

To get the crashing monorail, use the Twelve Train patch (see Hex Editing page).

To make the train go even faster, load the train with peeps (wait for full load, no max wait time)

If you want to be really cruel, use both the twelve train patch and load the train with peeps to have a 70 mile per hour, 70 dead peep, explosion. New trainers have made this creation obsolete, leading to even more fatal crashes.