While alternating current railways have been able to easily feed their braking energy back into the grid for decades, this was only possible with the development of modern power electronics for public transport companies operating with DC.
In the early years of this century, several companies began to develop substations that convert the braking energy backflow into alternating current in the substation, thereby enabling significant energy savings. The benefits are obvious, so there is no need to burn braking energy in rheostats, but it can be used by other consumers in the public network. This means for trams and trolleybuses waiving brake resistors or the heavy, short-lived and little environmentally friendly batteries that have to be carried onboard. In the case of underground railways, which often struggle with an excessive heat load in tunnels and at stations, the advantage is a lower heat load and thus secondary savings thanks to reduced demands on ventilation and, where applicable, air conditioning. For new systems to be built, another saving can be mentioned: the number of substations can be reduced by about 20%.
The high efficiency of up to 99% of modern IGBT power converters allows an almost lossless energy feedback and it is assumed that depending on the operating conditions up to 20% of the energy absorbed by the vehicle can be recovered. ATM Milano measured 15% at the first test on their metro line 3. Generally, it is calculated that the saved electricity costs will yield a ROI of about 5 years.
It is obvious that not every public transport company is willing to replace existing substations prematurely, so it will take some time for the market to fully develop.
The various manufacturers offer regenerative substations for voltages between 600 and 1500 V with outputs from 1 to 4.5 MW. It seems that Alstom is the market leader, so far he claims to have sold 128 Alstom HESOP substations in 10 cities.
An in-depth analysis of wayside energy recovery systems in urban railways can be found here: https://www.sciencedirect.com/science/article/pii/S2590116819300013