Tramways have a capacity that can sometimes be compared to a metro

The capacity of tramways, as well as other modes of transport, vary greatly depending on how capacity is calculated and the specific circumstances in the systems. Below are a few examples of capacity calculations. 

The public transport authority in Zürich, Switzerland (VBZ) is widely considered to have one of Europes best public transport systems. They claim that they are transporting about 2000 passengers per hour and direction with a 36 meter long modern tram (the Cobra). This can be compared to the alternatives, for instance a 24,5 meter double articulated bus transporting 1200 passengers, and a conventional 18,5 meter bus only transporting 900 passengers per hour and direction. 

Can transport up to 3600 people per hour...

In the above example the VBZ is only calculating the capacity using two passengers per square meter, compare to four which is usually used in comparison. The reason is that calculating with higher passengers numbers will increase the dwell times at stop causing longer travel times and delay risks. However, if you are using 4 passengers per square meter to calculate capacity, the capacity in a system with five minute headway will increase to 3600 passengers per hours and direction for a 40 meter tram, 2 148 for a 30 meter tram, 1320 for a double articulated bus and 1104 for a regular bus. 

...and in extreme cases you can reach 14 000!

But the capacity potential in tramways are even higher. In Budapest and other places 54 meter trams are used, which can carry 352 passengers with 1,5 minute headways. This means that you can carry 14 080 passengers per hour and direction. Spread out over the day (taking off peak into consideration) this means that 122 300 passengers travel each day, bring the annual total to 81,4 million passengers. This comparable to a metro system. 

Can be extended

When needed, trams can be extended. This can be accomplished either by adding modules to the tram, or coupling multiple trams together. Adding modules have the benefit that passengers can redistribute themselves in the vehicle with high loads, while coupling several vehicles together might reduce the need for maintenance.

In Germany the maximum length of trams that are running in the public road systems 75 meters.That provides a capacity of about 400 passengers per tram, using only one driver.