More capacity for the same cost is the case with double-decker commuter cars -- if they can fit into tunnels, that is. Bi-level train were once more common in the commuter trains serving NYC. However, these cars can have their own disadvantages.
Aug 20, 2010 The New York Times - N.Y/Region
The Federal Railroad Administration does not allow for the mixing of train types for safety reasons, which proved a key obstacle for Caltrain's plan for electrification. As a pilot project,they are allowing the line to utilize electric multiple units
May 30, 2010 Mercury News via Palo Alto Daily News