This might just be a nitpick, but comparing this movie with Midnight Run, in both cases the pair are unable to take a quick flight to L.A. In Midnight Run the next best alternative was to take the train to L.A. and only when that failed did they resort to driving. Any reason you can think of why Charlie didn't opt for going to L.A. by rail instead of going straight to driving?
Trains represent determinism, the preset path, the absence of personal agency. Driving, by contrast, represents personal efficacy. Their journey was of their own volition, indicating that their personal actions determined their trajectory in life.
In other words, they were masters (drivers) of their destiny and therefore in control, as opposed to being victims being carried on a path (railroad tracks) already set for them.
The title cover reinforces this idea. It shows them walking because they are in control of their lives, despite their imposed limitations. There is also the idea that personal development takes effort, and their journey throughout the film is that of self-improvement. This takes conscious effort, which is why we see the walking and driving symbolism, as they require personal action.
I like Fandango's take on this a lot BUT I think there's also a more practical, in story, reason:
they both wanted to travel in THAT car, their father's car that was Raymond's most persistent recollection of their father and, for Charlie, the symbol of the rift between himself and their father.
Once planes were ruled out, Charlie decided to use their "inheritance"
Yeah, while I realize trains are often symbolic, I think maybe they just forgot to throw in a short scene showing why Charlie chose to drive when he was so hellbent on getting back to L.A. Lingering resentment towards his father displayed by driving THAT car as much as he wanted now would have made sense.
Trains were not going to help with Raymond's regimented routine such as watching game shows at a given time when this movie was made. Also, his random outbursts might get them thrown off of the train.
Because then we wouldn't have had the fortune and pleasure of seeing Tom Cruise and Dustin Hoffman's character reconnect the way they did. The movie would've been an hour shorter if they took the train. What would he have done with the convertible if they took Amtrack??
Are you kidding? A 1949 Buick Roadmaster? The Queen Mary of the highway. Love to drive that machine from coast to coast. Just need a lot of $$ for gas. But what an experience that would be.