The only steam railway actually inside the New Forest: a 12¼-inch gauge line that loops for a mile and a quarter through the Rothschilds'' famous gardens at Exbury, near Beaulieu.
Exbury's little steam locomotives haul open carriages on a twenty-minute circuit through parts of the estate you can't otherwise reach — over a bridge, through a tunnel, past the ponds and the American Garden. It's genuinely lovely in late spring when the rhododendrons and azaleas that made Exbury famous are in full riot, and the railway runs whenever the gardens are open, roughly March to November.
Getting there from the rest of the Forest
Exbury sits on the quiet eastern side of the Forest near Beaulieu. There's no mainline station nearby — it's about a 20-minute drive or taxi from Brockenhurst station, and cycling there through Beaulieu is a flat, pretty ride. Check Exbury's site for garden opening days and current railway ticket arrangements before you set out.