The Marlborough Flyer operates for most of the year except winter, check this link for stunning pictures, timetables and tour options

www.marlboroughflyer.co.nz

Alternately if you just want to observe it and take pictures like these (click image to see video)

Click this map for the best viewing locations, Argyle Street (last one on the left heading out of town, park before the rail crossing) or Garden Terrace off the end of Devon Street, (go past the viaduct and look back at it)