I have 45 minutes to transfer from a train at Gare Lyon to a train at St Lazare (2 people, both with suitcases). It looks like the fastest way is by taking metro 14. Is 45 min. enough time to do this, or should I take a later train?
45 minutes is pushing it, especially since the Metro stations at major railway stations are notorious for long queues to buy tickets, whether from machines or ticket counters. I would suggest at least 60 minutes and preferably 90.
I vote for a later train.
You should be able to do it in around 30 minutes, platform to platform; if you don't get lost.
But, the first time there are too many risks, incoming train late, can't find the entrance to the Métro, queue to buy tickets (stuck behind some tourist who doesn't know how to use the ticket machine), can't find the way out of the Métro and up to platform level, finding the correct platform for your train at St. Lazare etc. etc.
Safer to just get a later train.