It seems like Stéphanie may be right, and it certainly seems to make more sense with "voyageurs". Alternatively, "trains" could be used if you wanted to stick to "voyages"...

I too would use "visitors" even if it is "voyages" in the source.

would interpreting it as 'voyageurs' be out of order?

I quite like the use of the word 'watch' here - obviously in keeping with the fact that it is a clock, and also the fact that it is 37 metres tall, which means that it is looking down on, or watching over, the travellers in the station.

Depending on the rest of your text, either "to keep watch on" or "to keep watch over" may be appropriate. "On" would imply a sense of monitoring/controlling (which I feel fits the original slightly better), whereas "over" implies that the clock takes on a more protective role.

I am not sure whether I would use travellers at all, and maybe translate as the comings and goings of the railway station. IMO the comings and goings, like voyages, refer as much to the people as to the trains.

http://idioms.thefreedictionary.com/keep watch
keep (close) watch (over someone or something)
to guard or care for someone or something. I'm keeping watch over my children to make sure they have the things they need. I think that an angel is keeping close watch over her to make sure nothing bad happens to her.

http://idioms.thefreedictionary.com/keep watch on
keep watch on someone or something
to monitor someone or something. Keep watch on the lady in the big coat. She may be a shoplifter. Try to keep watch on the committee's work.