All rolling stock to the Spa Valley Railway was delivered by road. The track with the Tunbridge Wells- Birchden Jn line was severed in 1990 upon singling of the Uckfield line. A temporary connection with the former down line between Birchden Jn- Eridge was made after the track at Eridge had been sufficiently disconnected from the then BR network to ensure that the lines remain seperated.
Lines that PinzaC55 mentioned such as Bluebell, West Somerset Railway, North Norfolk have connection through existing sidings or at the North Norfolk an end on connection.
At Eridge it is plain line --there is no points, no signals, no operational signal box. The nearest signals are at Ashurst Junction or Crowborough, The nearest signal box is at Oxted (15 miles away).
It is not only in BR times that Heritage Railways have been reconnected to the National Network in recent times connections have been made to-the North Norfolk (Sheringham), Bluebell Railway (at East Grinstead), Weardale Railway (at Bishop Auckland) and the Swanage Railway.
The Spa Valley Railway have received excellent help support from Network Rail , "Southern Railway" train operating company plus other Rail Contracting Companies and in my view far greater support than BR gave who closed the line initially.