not possible under these limits, with high containers the largest permitted, with the exception of high containers on some routes. Trials were made with the 4D double deck passenger train on some suburban lines, requiring alterations to overhead bridges and structures, but no double deck trains are used today. Signalling Victoria uses a mix of railway signalling practices: British route signalling with home and distant signals (2 position signalling) and American speed signalling (3 position signalling). Semaphore signals were used on the very first railway lines, but only a bare minimum were provided as the time interval system being relied upon instead. The first interlocking of signals to protect trains was provided in 1874, as before this time conflicting moves could be made. The design of the signals also progressed, with the disc type siding signals first introduced in 1885, and the lower quadrant somersault type main line signals adopted in 1887, both of which are still in use today. Green was not adopted as the All Right colour until 1898, with white being used before this time. Red was the usual colour of all signal arms, until yellow was chosen as the colour for distant signals in 1926, with full adoption made in 1930. Colour light signals first appeared in 1918, and by 1924 they were the standard for new installations. The safeworking of trains between stations on the early lines was time interval working, where a train would be allowed to leave a given time after the train before it. With heavier traffic this method became unsafe, with Staff and Ticket working on single lines adopted from 1873, and telegraph block working from 1878 on double lines. Both of these systems ensured that only one train would be in a section of track at one time. Telegraphic block working was then replaced with Winters Block working between 1883 and 1888, a system that is a predecessor of the Double line Block system which is still used today. Later years saw variations made to the Staff and Ticket system, with busier lines provided with Electric Staff working which provided greater safely when more trains ran. Heavier suburban traffic on the Melbourne network saw a greater strain on the block working then used, which required a large number of manned signal boxes to enable trains to run close together. As a result, it was decided to adopt power signalling under