How long of a layover would justify leaving the train station to do some walking around the city? I suppose it depends on the location of the station. Any thoughts on a 1.5 hour layover in Marseille? Does anyone intentionally schedule 2-3 hour layovers in cities or is it usually too rushed too accomplish anything?
"I suppose it depends on the location of the station." There you have it exactly. I'm not familiar with Marseille, but in general, it is usually much easier to fit in some site-seeing with a train layover than with a flight. The stations often sit in or close to the city centers. You can literally arrive at the station 5 minutes before a train leaves and still catch your connection
According to www.viamichelin.com, it's 2km from the train station to the old port. It would be about a 20-minute walk each way.
You will have time to get a feel for the city. Even if I only have an hour, I try to walk around every city I visit. Obviously it may not allow for any in depth visiting, but then again with a little research you may want to focus on one specific sight. I really enjoy visiting Cathedrals. If this is of interest to you, check out Notre Dame de la Garde. You not only get to visit a great cathedral, but you also get a phenomenal view.
I would take a taxi or check out public transportation. There is a metro stop also called "Notre Dame de la Garde" and bus #60 goes up there and stops right in front of the church; however, if time is vitally important the taxi is the better choice. The views of the port, the city, etc. is really wonderful, but then you also get the pleasure of seeing this wonderful cathedral. As I recall it has a strong influence for sailors. The original church was dedicated to Our Lady of the Watch (or similar wording). The architecture is stunning and it is worth a visit. Cheers.