What is under/below the bed?

Al

I once grew two Roma tomato plants in a garden box that was 10" wide, 30" long, and only 6" deep. They were left over seedlings that I couldn't bear to throw away. The vines got pretty tall and I had to tie them up to a screen for support, but they did quite well. I imagine a determinate plant would do much better.
If you are using a regular raised bed and not a box, I would dig down below the growing medium to loosen the soil beneath, because it's always better to give the roots as much room as you can. Hope this helps.

Yes, you can, even if the bottom is cement. There is no reason why the roots need to go downward if all the water, air and nutrients they need are in the 5.5" you have. (In practice this is a fairly significant if.)

The Square Foot Gardening method uses 6" of potting mix like soil and some, such as gardeners with difficulties bending down/over have them set up on stands to the garden is at waist level. The tomatos grow very well, you just have to support their above ground weight (like any tomato).

Thank you all for your replies! Yes! I had forgotten about the square foot method and the enclosed garden box for vegetables. My raised beds will be on the ground (I can loosen the bottom underneath) so they'll have a little bit more room there. It's hard to be believe a 4-5 foot tomatoe plant can grow from a 5 1/2 inch raised bed.

Thank you all for your help!

Also something else to think about is planting your tomatos in a "trench". I pinch off all the leaves (except for about 2 sets on top) and dig a trench and lay the tomato down in the trench (at an angle) so that when I put the soil back, just the top of the plant is above the ground. The entire planted "stem" will become rooted :) I've had great luck planting tomatos this way in my square foot garden.