While investgating ways of getting from CDG to Caen, I came across a note that Gare St Lazare is being worked on and trains may not be running? Any information on this?

When are you talking about? That station had a lot of work done on it a couple years ago, I think it ended early last year. Trains are running from it right now, this is the status

http://www.sncf.com/fr/gare-de-paris-st-lazare-FRPSL

It's not the Gare Lazare that's being worked on; it's the line between Paris and Caen/Bayeux. I believe you can still get there, but it's taking about 4 hours and a couple of connections.

http://www.ter-sncf.com/regions/bass...x/default.aspx

It's not the Gare Lazare that's being worked on; it's the line between Paris and Caen/Bayeux. I believe you can still get there, but it's taking about 4 hours and a couple of connections.>

Current schedules indicate less than two hours direct St-Lazare to Caen so about normal and no changes of trains needed - now with this type of track work it is often tried to be done at night or during slack train use so there may be some times when it could take 4 hours and have to change but at least currently I can see no Paris to Caen trains taking that long or requiring changes (fast trains) - this comes from www.bahn.de/en - German Railways pan-European web site that usually is infallible IME.

anyway nothing to worry about.

Worth reading what the actual train operator has to say about it at the link St Cirq

..gives, rather than another country's rail system website, wouldn't you think? Especially since TER-SNCF spells out what's being done, where, and on what days.

The link (plus www.voyages-sncf) shows work going on until April 20. A few days ago there were perturbations on most of those trains so that many routes were taking extra long and involving changes of trains. I think it's changing day by day as work progresses down the line. I've checked several random days, and some seem normal, some not. We don't know when the OP wants to travel, though. I believe there is further work expected on that same line later this summer, too.