Anything you build should have a good foundation to start with. Much of the work you do on your train layout is going to be tedious, and it's going to be important that you have a solid foundation to work on. A card table or anything that moves around and can be easily knocked over is not what you need for your very involved hobby. You can build your own bench work, which is highly recommended. It doesn't take a lot of carpentry skills to measure, cut, level, and nail lumber together. The best part of building your own bench work is you can fashion it any way you want it. Of course, the room where you will put your setup is an important part of the size and configuration of the bench work, but you can make a perfect fit if you build your own.
How your bench work is set up has a lot to do with maintenance and potential accidents. If you have a setup which has access on three sides, it makes it easier for people to view it from more than one angle, however it makes the train layout much more susceptible to someone knocking it over. If you build the bench work to fit against the wall, you have something to secure the work space to, and it only leaves one side from which you can access the work area, decreasing the chances of an accident. Another good reason to secure the setup to the wall is that a back drop of sky or whatever you choose can be painted on the wall making the setup more realistic. It is generally better to finish the top of the bench work to a height that makes working on it comfortable. Another important consideration of height is to keep the setup high enough off the ground to prevent animals and small children from accessing it. A good rule of thumb is about 36" because it is about counter height and it can be accessed while standing.
Bench Work Materials
Although you can build a bench out of almost anything you want, it is better to make the top out of thick material. A 3/4" thick piece of plywood is much better than settling for something thin because there will be less give between supports. The framework can be built out of 2" x 4" pieces. Horizontal support framing should be no more than 16" apart, and it is even better to place these supports on 12" centers. When the wall is used to attach one side, there's no need for support legs except on the outside. Attaching to the wall also eliminates the need for cross bracing.
It is better for electrical outlets to be located beneath the bench work so they are not a part of the scenery. It is not required, but having lighting directly over the set is good for appearance, and it helps when working on small components. Fluorescent utility lighting hung from the ceiling is a good choice. It is an advantage to have drawers under your bench work for storing tools and related railroading materials for easy access. The most important thing about your bench work is for it to be comfortable, functional, and convenient for you. You will spend many hours perfecting your layout and enjoying your trains, so please yourself however you choose to build your bench work.