Hi C.! What a great age!!! In this year alone (before he turns 3), you'll go from barely comprehending him to understanding his full (broken up, cryptic) sentences in a span of 6 months. Two-year olds spans a wide range of both development and skill. Two year olds are just beginning to really understand the world around them and I have found they love to pretend and make-believe. My oldest son's imagination soared at this age; I always found him in his room with either his stuffed animals or plastic dinosaurs acting out scenes. They also love dress up and being silly with hats and scarves and just about anything else. Try also challenging them with wooden puzzles (10-20 piece) and big blocks (the brands 'Melissa and Doug' and 'Imaginarium' are awesome and reasonably priced). Play dough is also a great pasttime (you'll find him sitting there for at least 1/2 hr by himself). Try finger painting, listening to fun music and dancing (Wee-Sing has awesome CDs). This is also a great age to start playing "Simon Says".
Like yours, my middle son loves trains and cars. He's also more athletic. At 2 years, he was already climbing the monkey bars by himself, loved the swings and insisted on sliding the "big boy" slide at this age. And, interestingly, as much as he enjoys anything that rolls, he is really fond of animals, too. So you may want to give him a toy that provides exposure to animals (Fisher Price's Little People's Noah's Ark could do just that). Then you might find, he may actually be more interested in animals right now.
Either way, his short attention sounds normal. I believe that children at this age are always looking for something to learn and absorb. Their mind wants to be challenged - to dissect and put things together. At this age, self-accomplishment is important and can do wonders. The trains and trucks are wonderful for his imagination, but he probably wants stimulation outside of his own creation.
Keep listening to him (always), be patient (with yourself, too), and have lots of fun!