I wanted two lines, running off my main, to use as storage. The idea was to have two trains ready to run with just the addition of a loco.
I built a tunnel with a stone entrance portal , it is 8ft. long and contains two tracks. It is a dead end tunnel but can be extended (if necessary) at a later date. This tunnel is beneath two larges bushes and is not really visible, except the portal. It would not be a suitable area for open track as leaves and access are not easy. The tunnel, built from roofing joists as sides and laid upon 20" square slabs supported by concrete blocks, has a removable roof - just in case.
This has worked well and has given me a storage good feature which was not possible in other areas of the garden. I have since made another storage tunnel in the opposite part of the garden and a storage line leading to another dead end tunnel.