This type of ABS setup strikes me as "glorified automated signalled Manual Block". I am not familiar with the Southern Pacific lines being described, but I gather from the description above that the distances between sidings are rather long -- i.e., that the distance from the overrun signal to the distant signal is much greater than the distance from the siding-exit signal to the overrun signal, or from the distant signal to the home (siding-entrance) signal.
Given this, let me explain my concocted term above. This arrangement appears to run much like Manual Block in that the most direct control is exercised over the section of track between sidings. Other lines, particularly in the eastern US, that may on the surface appear similar, really exercise control over the short section of track around the switches as I hope to show soon on a page describing CTC.
It is "signalled" for obvious reasons. Note the similarity to European signalling: the siding-exit signal is similar to a starter signal, and the distant and home signals have similar functions. The difference between SP's ABS system and European signalling, and between SP's ABS and true Manual Block, is that the system works without human intervention; thus my term "automated". "Glorified" was added because of the overrun signals. These signals would not be used in a plain vanilla Manual Block system, since in that system, signals are operated manually and protection is provided manually.
Thus, SP's ABS begins to approach "eastern US ABS" or CTC in degree of protection. The latter two systems provide a finer degree of train-spacing control between sidings, in addition to direct control of signals at the beginnings and ends of sidings.
Text, Images, HTML: Carsten S. LundstenLast updated: 1997-11-19