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.

As regards the incorporation of anglicisms and despite the efforts of the Real Academia Española (an official regulatory body which aims at the purity of the language), Spanish has usually been considered a rather permissible language.