Definition of "unlined"

To keep constructional costs to a minimum track formations were narrower than is now customary, many tunnels were unlined, timber was largely used for bridging, and 85 lb. per yd. rails were regarded as adequate.

1941 March, “Notes and News: Modernising a Main Line”, in Railway Magazine, page 133

[...] all other classes are painted in unlined green.

1941 January, “Notes and News: Locomotive Notes”, in Railway Magazine, page 45