IMO the original place known is also the easiest to do:
At the last riverside support column for the L-Train you see *before* the concrete ramps upward, push a large vehicle in the water. Use a motorcycle to land on top of said floating vehicle...then exit your 'cycle. You will drop down to the subway ceiling below...and you can exploit this glitch to gain early access to the other islands.
NOTE: it's also easy to get random vehicles to fall through if you stand over at the subway loading platform. The tunnel gfx will not disappear if you are using a vehicle to drive away from a loading platform.
Similarly, you can use a motorcycle parked in a boat to drop down. IIRC the Speeder near the Callahan bridge in Staunton is just above the subway tunnel.
Also, the Porter tunnel that exists between Francis Int'l. and Wichita Gardens can be dropped onto by using a large vehicle with a high seat placement (such as the firetruck) to "dip" below the water surface near the stadium (i.e. cab under water, rear wheels still above ground). Exiting the vehicle in this manner fails to detect that your character is on a water surface - so you fall through and do not drown.
Squeezing yourself between a barrier and a vehicle may allow you to beat collision barriers (such as jumping through the Porter tunnel ceiling using an ambulance parked on either sidewalk to stand on). This glitch is far easier to perform in previous titles...but it is still present here too.