Both usages occur; for example, in Boston (Massachusetts, USA), one refers to the lines of the “T” by color, so you might take a train on the “Blue line”; similarly in London (England), where the lines have names, so your train might be on the “Bakerloo line”, and in New York (New York, USA), where you might be riding on the “A train” or “7 train” - but in Toronto (Ontario, Canada), the lines are referred to by number as “Line 1” through “Line 6”.

Both usages occur; for example, in Boston (Massachusetts, USA), one refers to the lines of the “T” by color, so you might take a train on the “Blue line”; similarly in London (England), where the Underground/tube lines have names, so your train might be on the “Bakerloo line”, and in New York (New York, USA), where you might be riding on the “A train” or “7 train” on the subway system - but in Toronto (Ontario, Canada), the subway lines are referred to by number as “Line 1” through “Line 6”.