Hi Chris,
by train you have to go to Krhanice. It is on the opposite side of Sazava river of Lesany. From Krhanice train station it is circa ten minutes on foot across the bridge to the museum.
From Prague Main Station goes train nearly each hour there.
http://www.idos.cz/ConnRes.asp?tt=c&from=1&from=2&from=36035&froml=-1&to=1&to=0&to=3899&tol=-1&date=24.6.2007&time=0:17&min1=5&min2=0&max1=240&max2=240&alg=0&isdep=1&beds=0&chn=5&stop=0&odcht=0&odch=50&link=96DA&prev=0&handle=167850
Museum is open for public in June and September on saturdays and sundays from 9 am to 5 pm and in July and August each day without mondays from 9 am to 5 pm.
http://www.vhu.cz/cs/stranka/vojenske-technicke-muzeum/
Regards
Tomas