Birmingham and Bristol Railway
At Gloucester the latter had formed a junction with the broad gauge Cheltenham and Great Western Union Railway running into the town on mixed gauge tracks. In 1843 the C&GWU had been taken over by the Great Western Railway which began putting pressure on the Bristol and Gloucester to join the GWR at Bristol, to subscribe to the proposed South Devon Railway, and to convert to broad gauge track, to the alarm of the Northern "narrow" gauge railways.
The "break of gauge" at Gloucester was a major problem. It caused pandemonium as whole trainloads of passengers, and their luggage, changed from one to another, together with the transhipment of goods. Parliament had established a commission to examine the problem and there was a consensus that the track should be unified throughout the line. The GWR made an offer to the Birmingham and Bristol directors. The latter's shareholders held out for more, and the GWR deferred its decision for three days. At this moment the Midland Railway made its move.
True or not, the story of how it came about is a part of railway legend, when the Midland's James Ellis was travelling, quite by chance it is said, in a train with two Bristol and Gloucester directors. He overheard them discussing the matter, and took it on himself to offer better terms. In 1846, it become a part of the Midland Railway.
The Midland's great rival, the LNWR, was so alarmed at the idea of the GWR's broad gauge reaching the Midlands, it had offered to share any losses the Midland might incur. In the event, all that was needed was a nominal rent for the Midland's use of the LNWR's New Street station in Birmingham.
There were, of course, still problems with gauge. In 1848 the Midland built its own line into Gloucester, avoiding the GWR ex-Cheltenham & Great Western Union line, and laid mixed gauge to Bristol. By 1857 the whole line had been converted to standard gauge.
Sources of Information
-  Wikipedia