Status quo in DC local networks:

~ 40% of energy demand goes into kinetic (or potential) energy and could theoretically be recovered.

If vehicles are equipped with regenerative brakes, 10-30% are recovered and used by other trains (in dense networks). The rest is converted into heat in on-board resistors.

The installation of stationary energy storage systems is an interesting option on peripheric routes with low train density and/or high velocities and slopes. In such areas of the network, a direct use by other trains is rare, so recovery rate will usually be < 10% (of total energy demand), leaving > 30% of theoretical potential for energy savings through stationary energy storage. There are however further limitations:

Losses in energy transmission, conversion and storage

Layout of storage system is not sufficient to store the entire braking energy (e.g. in the Cologne case, the average braking current is ~1000A, the flywheel can only take 450A). The layout is usually a trade-off between technological and economic considerations

In the Cologne case, the average power demand of the substation equipped with a flywheel was only 160 kW compared to 210 kW without energy storage. This means energy savings of about 24%. On has to keep in mind that these savings were realised at a carefully chosen location and can by no means be generalised to an entire network.

The system-wide effect achievable by putting storage systems at all “hot spots” of the network is very difficult to assess and strongly dependant on the specific network layout. Given the above figures and the limitations mentioned, it seems reasonable to assume that the overall effect for a local DC network could be between 2-10% depending on the density of "hot spots".