By googling for railfanning in montreal ( see my previous post ), I found that the AMT ( Montreal Metropolitan Transport Agency ) organises this summer 3 railway roundtrips from downtown Montreal to the Canadian Railwau Museum in Delson.
http://www.amt.qc.ca/escapades/2007/express_musee.asp
http://www.amt.qc.ca/escapades/2007/express_musee.asp