(2-part post):
The official Kranjska Gora website states that the nearest rail station is 20km away, at Jesenice:
http://www.kranjska-gora.si/index.php?i=139
However, the Thomas Cook Rail Map of Europe shows that one of the two possible rail routes passes through Tarvisio, so your plan to collect your friends from there seems to be a good one.
The best website for European rail travel is that of Deutsche Bahn, which includes all European rail timetables in its database:
http://reiseauskunft.bahn.de/bin/query.exe/en
Alternatively, the Trenitalia site is here:
http://www.trenitalia.com/en/index.html
There's no rail station at Marco Polo airport. You have to take a bus to Venice-Santa Lucia station (which involves a 10 minute walk across the Ponte Degli Scalzi) or to Mestre-Venice station:
http://www.veniceairport.it/page/servizi/trasp orti/treno?m=01020201&language=en
The train service seems to be very irregular and journey times vary between about two and a quarter to three and a half hours.