TT and RCT do look very similar, visually, when you play them, and the GUI seems to be very similar too. Being a programmer, I reuse my code a lot if it's good code, so I can imagine Chris Sawyer reusing some of his old graphics and gui code, altering it along the way, but recycling it nonetheless. But keep in mind all the stuff you would need to do to turn TT into RCT. First off, you need to prerender a truckload of 3d models at different angles, somehow working the palette into an interchangeable one so you can change the colors and stuffs. Not to mention, RCT has rollercoasters, whereas TT has cars and trains and boats and stuff, so I'd imagine TT woulnd't have a whole lot of suitable physics programming built in to power a rollercoaster simulation at a satisfactory quality and grade of realism.
TT -> RCT isn't impossible, but the work it would involve is very great, and it would take a very dedicated team to handle it, especially considering they'd probably be working for free (open source, right?).
It would be impossible to replicate RCT exactly (duh), but one probably could create a similar game.
But with all that headway, workload, interpreting the code, understanding how the routines work, etc, why not just create a clone from scratch?
