center tap transformer full wave rectifier current rating
You can safely draw about 5A from a 48V CT 5A(RMS) transformer (rated at 240VA) with a full-wave (2-diode) rectifier and filter capacitor. 

The voltage will be about 33V peak, so about 170W total (including rectifier losses) with a large capacitor. 

With a linear regulator you will get 5A out of it, plus about 50W of waste heat. With a buck switching regulator you could get perhaps 6A depending on the efficiency of the regulator. 

All in all, though you'd be better off buying a decent brand of commercial off-line switcher that can supply 24VDC regulated at 7A or 10A or 15A for most, not all, applications. A 14.6A (350W) supply is less than $50 US, almost 90% efficient, and weighs relatively little.