The closest railway station is Aviemore. There is a direct train from London that takes a little over 7 hours. A change at Edinburgh is likely from other destinations.
The Caledonian Sleeper is a wonderful way to travel overnight - full details, including timetables, prices and pictures, can be found on www.seat61.com
Car hire is available in Aviemore at the railway station: www.aviemorecarhire.co.uk
Another option is to take a taxi from Aviemore (there are plenty of shops in Aviemore, including a Tesco, and it makes sense to do a grocery shop first). A taxi to the cottage will cost in the region of £40 one-way.