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3-aspect vs. 4-aspect: Yes, some ABS systems do not have the fourth aspect -- in fact, most pure ABS systems on single-track (that is, without a CTC overlay) are only three-aspect. The only difference is the lack of the flashing yellow. Signal spacing may have to be adjusted to provide adequate stopping distances. Single-track ABS systems without CTC are becoming uncommon these days -- the Santa Fe passenger line comes to mind. <br /> <br />Capacity: Directional double track has more capacity than single-track CTC in the ideal case. But it's not that simple, of course. Taking one track out of service for maintenance results in serious train delays. A railroad with a broad mix of train speeds, and a lot of traffic, requires either outside (Santa Fe practice) or center (UP practice) sidings for runarounds. You can use these as very rough rules of thumb as maximum reasonable capacity: up to 35 trains/day, single-track CTC; up to 60 trains/day, double-track ABS; up to 100 trains/day, two main track CTC. Go much over those numbers and it will be pretty difficult to make it run with fluidity, depending on geography, train types, etc. <br /> <br />Upgrading from ABS to CTC: unless the ABS was designed and installed with this in mind, it's pretty much going to be mostly thrown away. You'll save the hardware and the pole line, but it will all have to be rewired and probably the signals respaced, too. Labor is the big expense. Many late ABS systems were designed to be upgraded to CTC (notably the Moffat Road).
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