OK, did some web digging. Turns out that Wayside Energy Storage has been done, experimentally, in several areas around the US; however, the amount of documentation available is small and not consistent from project to project. Energy storage modes used include batteries, super/ultracapacitors, and flywheels. The systems for which I found references to using it (or at least trying it) include NY subway, SEPTA, LA Metro, and Sacramento. It appears that Toronto also considered it, but may not have done anything about it yet. SEPTA is by far the most prolific in its PR. A few examples:

SEPTA (Philadelphia):
Historic Project Brings Wayside Storage to Scale (1/21/16 blog)
Wayside Energy Storage Project - Progress Update (7/15/2011 blog)
That's the brakes: Utilizing stored energy from public transit for grid services (UtilityDive, 2/16/16)
Supercapacitor-Enhanced Hybrid Storage to Earn Cash for Subways (IEEE Spectrum, 4/14/2014)
How a 'dinky' energy storage project gives financial oomph to Philadelphia's subway system (E&E ClimateWire, 6/6/2013)

Others/PDFs:
(Theoretical) Wayside energy storage study. Volume II. Detailed description and analysis. Final report May 1977-June 1978 (Researchgate)
APTA White Paper (2012)
APTA Powerpoint (2010)
TCRP Web Document (2010)
DOE FY2007 Projects Powerpoint (scroll to p. 11 for Sacramento RT)
TRB: Advanced Wayside Energy Storage Systems for Rail Transit (2013)
LA Metro Wayside Energy Storage (2014)

There are undoubtedly other papers and stories out there, but these are what I found in a few minutes searching that, on quick review, might be relevant to trains (or at least rail transit). The original item for this thread is a different story...