Commuters wondering why trams aren't running more frequently on Sydney's inner west light rail line during peak hours despite a boom in passengers can now blame "capacity constraints".

Internal government documents obtained by Fairfax Media show constraints such as "power supply, stabling facilities, single track near Dulwich Hill and fleet size" are limiting a significant increase in services on the line from Central Station to Dulwich Hill during the morning peak, when overcrowding is at its worst.

Of an extra 95 weekly services announced last week by the state government, fewer than a third will run during the morning peak hours of 7am to 10am. The rest will operate during the "inter-peak" period of 10am to 3pm, and at weekends.

Patronage growth on the line from Central Station to Dulwich Hill has been "exceeding all expectations" of transport planners.