The train should be the most convenient way to get around Istanbul, but in practice the system is fragmented and can be confusing. Completed in 1875, the Tünel is the second oldest subway line in the world, behind only the London Underground. It connects Karaköy and Beyoğlu around Istiklal Avenue.
The B1, M1 and M3 are commuter rail lines on the European side, starting in Eminönü and extending westward along the Mediterranean. The B3 is the commuter rail line on the Asian side. The M2 is the first line of the Istanbul Metro. It runs from Galata up through the Levent business district. The second line, the M4, is currently under construction on the Asian side. There are also two funicular rails and trams that traverse some of the city's steeper hills.
The cost for a single ride is generally around 1.50TL. To transfer to another line or to the bus, you'll have to pay again. The lines are open from 07:00-21:00 Monday through Saturday.
Note that all public buses, trams, trains, and sea buses accept the AKBIL. There is a refundable 6TL charge for this device, which is much more convenient than buying tokens. Rates are slightly discounted and it can be refilled at booths in all the major stations.