Definition of "four-track"

[...] 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