Full rules are here, but basically there's a distinction for travel within instead of between cities: you can use taxis, a hire or folding bike etc. to get around locally. This seems fair as most likely you'll just backtrack to the main transport hub to continue your path anyway, so it would be frustrating having to use public transport to your hotel, for instance.

Those rules were drafted when checkpoints were cities, so a clarification was made early in the race to confirm it covered the dam too. Since it's enough to get to Tenero station anyway, it seems unfair to penalise someone for taking a side trip (by taxi) before resuming the race.

Nonetheless, I'm doing it without taxis - mostly because I just find them too expensive and awkward to arrange!