In thinking about Paris and New York in the context of Alon Levy's guest posts at the Transport Politic (one, two, and my reply, and Jarrett's), one of the big differences that struck me between the two commuter rail networks is the approach tracks to the stations. Paris's are huge: the six main line stations had between six (Austerlitz) and fifteen (Nord) from the city limits to the stations. This number of tracks continues until the various lines branch off. Only the small lines had less: four for Invalides, two for Vincennes and two for Sceaux.
By contrast, here in NYC, we had a maximum of four tracks at five stations: Grand Central, Penn (for the LIRR), Bay Ridge, Hoboken and Jersey City. All the rest have or had two, and the Empire line has only one track over the Spuyten Duyvil bridge.
In the comments to my previous post, Adam writes, "in order for a rail terminal to be effective, it must have six tracks going into it at least." If he's right, it's not surprising that commuter rail in New York has been less effective than in Paris. It also points to the benefits that Alon's plan could bring to commuter rail in New York.
However, I will point out another difference between Paris and New York. There were some lines that were entirely taken over by the RER, like the Sceaux line, and their central terminals were converted into simple stations on the new lines. Most of the RER is made of lines that used to terminate in one of the six main-line stations, however. Some of them were rerouted into totally new alignments within the city: for example, the Saint-Germain line was run through the RER A tunnel instead of to the Saint-Lazare station. This was essentially what the Pennsylvania Railroad did in 1910. The rest were diverted into new through-stations built underneath their former terminals, such as the RER D, which connected lines from the Nord and Lyon stations. This is more or less what the LIRR East Side Access plan is doing under Grand Central, except that the line isn't going through.
The result of this is that in Paris the capacity of the old stations is pretty much untouched. Every new RER line represented an indisputable increase in central capacity. However, Alon's plan takes capacity from the existing Penn Station trackage for the through tracks. I think that the increased capacity from through-running would more than make up for it, but I felt it was worth noting.