Definition of "permissible"

[...] Mr. M. N. Rollason points out that on four-track lines on which the fast lines, in the centre, are flanked by the slow lines, and running at speed is permissible on all four, the traveller can enjoy some quite exciting experiences when trains are doing a "neck-and-neck" on adjacent lines.

1944 May and June, “Notes and News: Express Travel on Slow Lines”, in Railway Magazine, page 184

The units will be allowed to run up to 90 m.p.h. between Chelmsford and Colchester, but their maximum speed of 100 m.p.h. is not permissible on the G.E. line.

1963 March, “The Clacton express electric multiple-units enter service”, in Modern Railways, pages 172–173