Regen ... but you have to have a load. A single-track line with light traffic might not often have another train in the electrical block that can receive the generated power, in which case you either need a way to store it or you have to waste it in normal dynamic braking fashion someplace - either on the loco or at a substation.

Sacramento's light rail is a case in point. The CAF cars are equipped for regen braking (the old Siemens are straight dynamics), but the substations (most of them) are old, and there often isn't another train accelerating in the short blocks they have when a slowing train would be generating power. So as I understand it the regen function is currently turned off and the trains just waste power to resistor grids in the normal fashion (you definitely smell it at times!). They've talked about setting up substations with ultracapacitors or the like to absorb the regen power and supply it to another train later. AFAIK it hasn't happened yet, or at least not throughout the system.