No this is from the tow path. Or coming from Casisobury Park, you cross the large bridge over the canal, and can head right or left along the towpath. Well, there there's a sign to Whippendell Woods, but there's a white sign saying Cyclists Dismount. The lane itself is a little dirt track through some woods. I'll try and find it on Google Maps.
Edit:
Can't find it, which is understandable as they couldn't get their little streetcar up there.
Ahh well, maybe I'll take the track next time anyway and see what happens.