Tube is a little known game from Bullfrog which, according to Home of the Underdogs (see links) was originally intended to be a test for a new Bullfrog employee.

A unique concept in itself, Tube utilizes an old demo trick to simulate an endless forward-moving tunnel, along with some neat vector effects to simulate a long race track of sorts. In a nutshell you have to pick up speed in order to beat the track before your time runs out, you also have to destroy various obstacles in your way and collect weapon upgrades.

This game was never made into a title by Bullfrog and was released as a freeware game -- see trivia.