I have been commuting to work for some years now. I prefer to work during the journey instead of having to steer the car myself. For my comfort I therefor invented this light weight collapsible table. The table has two different ends to accommodate the two different types of tables the bus company uses.
I will try to make this instructable as generic as possible, so you can make a table that matches the table in your bus/train...

Step 1: Taking Measures

First you have to take measures of the table you need to match. I used some tinfoil from my lunch box to measure the thickness and depth of the table and to figure out, how high the edge of the table is. I folded the foil around the table edge to make an imprint.

Step 2: Constructing the Table

To make the construction drawing I drew a side view of my Mac Book 17" on a piece of paper. Then I drew the supporting structure of the table around the tin foil imprint. After some experimentation, I figured out, how long the lower support structure had to be to ensure the PC table would have the right angle.

Step 3: Cutting Out the Table Pieces

I used 12 mm plywood to make the pieces. I removed a lot of unnecessary wood from the structure to make the table as light as possible. The cutting was done with an electric jigsaw.

Step 4: Testing

Testing the table turned out successful – yay!!!

Step 5: Following Instructable

So now I can let the driver do his driving allowing me to read all the nice Instructables on my way to work...
I hope you find this instruction helpful – have fun!