Need to be careful with terminology. CBTC is a term usually associated with certain types of signalling used on the underground, and Crossrail of course. Shorter block sections allow shorter technical headways and therefore improved planning headways and thus higher frequency, up to a point, limited by braking distance between signals and linespeed. ETCS (or similar) cab / radio based signalling systems enable a further reduction in block sections, not limited by braking distance and linespeed, and this does enable a small improvement in technical and planning headway and therefore frequency, but usually only at lower speeds and plain track. ATO removes any variability in driver behaviour, and also enables a ‘braver’ driving style, ie braking later than a driver would for a station. In effect this allows a reduction in the ‘margin’ between technical headway and planning headway, which further improves frequency.