Tunnels being naturally dark may not be the most intuitive choice for a place to take pictures. But I love the 'painting with light' work that artists such as 'Twin Cities Brightest' are putting out and they're clearly seeking out places like tunnels, caves, garages... Dark places in other words, to create their works of art. Which brings two questions to mind:
Any scene with isolated elements much brighter than their surroundings is fair game. Expose for the bright lights so that everything else is suppressed and go to work. A 1/4 to 5 second exposure works well: it gives you enough time to do interesting things but not too much to really screw it up ;-).
My favorite, since I first got a decent camera as a child, has been the annual Christmas tree. You can shoot it out of focus:
(Front focus and back focus give slightly different shapes to the bokeh. The size is determined largely by how out of focus the image is, but it depends on the lens and the aperture too.)
You can move the camera around during the exposure:
Zooming during the exposure gives a characteristic effect:
You can do these in combination, such as this one that was unfocused while zooming:
Use your imagination and have fun!
Incidentally, it can be difficult to stop down a brighter scene. These images were taken at ISO 100 with apertures around f/11 for 1 - 4 seconds in a darkened room. An ND filter would be your friend with brighter lights (such as daylight at the end of a tunnel).
Not directly related to photography, but to techniques regarding Light Painting: You might also want to take a look at Light Painting for Video. Instead of using long exposure, you take a video and apply the effect.
The page linked describes an effect I have written, there are also other implementations, but to my knowledge these are not public (i.e. kept at home in a treasure, kind of).
Painting with light takes time; you'll need a place where there's not much ambient light to draw out surroundings while your shutter is open. Many of the effects can be carried out in a studio, or even at home in dark.
In city, outdoors is generally not suitable for all but shortest tricks because there's so much light pollution from streetlights, cars, billboards etc. You might succeed somewhere under a bridge (see your local map) or near/inside an abandoned factory/warehouse. Tunnels are usually lit for security purposes and therefore useless for light painting.
Outside city, you could ask/look around for abandoned buildings, and you might get lucky finding a non-illuminated tunnel.
Some basic techniques.
If you are in dark clothes and avoid getting between a lit surface and camera, you may walk around on scene without getting caught on the frame. You'll need that to move your lights etc.
The technique I was taught in a light painting workshop: use a handheld flashlight to "paint" an object in dark. You'll get an uneven lighting; you can stress some areas by holding your light on them for longer. Example made using a pen flashlight:
Fake multiple exposure by painting part of the object, turn off your light, change the scene, continue painting. For example, paint a door, then open the door and paint something behind the door.
Use different colors of lights and move them through air to create patterns. You can use gels to color your lights. A very simple example, I just waved a gas lighter in a tight range:
Attach multiple lights on a rigid frame to draw parallel lines or concentric circles.
Make a stencil, a box that has a shape cut into a side and covered with translucent material (paper will do). Use (color gelled) flash to create those shapes in your "painting" by flashing into the box with the shape towards your camera.
For more complex pictures, usually multiple lights and techniques are used.