The main disadvantage of multi-level cars for Metro-North is that they would not be self-propelled (also called electric-multiple-units, or EMUs), each receiving electricity from either a third rail or overhead catenary, as there are short gaps in the third rails in the maze of tracks under Grand Central Terminal, and potential solutions, such as diesel/electric locomotives, have their own disadvantages.
"Two-level trains cost the same as their single-story counterparts, and can carry a third more passengers." The trains could also fit in the tunnels.
"Double-decker trains, then called "up and downs," were commonplace on the Long Island Rail Road from the late 1930s to the late 1960s; a more modern version began operation in 1998. New Jersey Transit has run double-deckers since 2005.
Officials at both commuter railroads say the bilevel trains receive rave reviews from customers, particularly because their interior layouts eliminate the hated middle seat."
Note: Article also reports on the bar cars that Connecticut is consider adding to its fleet, noting that plans have changed from purchasing new cars to retrofitting existing cars.
Thanks to NYT-commuting-transit alerts