[Aeolun]

I don’t think the original RCT would let you build underground at all? Seem to recall that starting (or becoming practical?) from RCT2 onwards.

[arkadiytehgraet]

Original RCT most definitely had underground building, both for paths and attractions. I think 1 or 2 parks in the 'campaign' was even centered about being mostly underground.

[chippiewill]

You could definitely build underground in the original game.

I only ever had the original one when growing up and did stuff underground a fair bit. One of the early campaign parks came with a bunch of rides and stuff underground off the bat.

I think there were some practical changes to make it a bit easier in RCT2, but it was never hard.