Trains4fun100
Hello & Welcome to the Forum!
Some train fanatic's refer to that dirt as natural weathering or sign's of love shown to the train..  lf the train has been played with/loved.  NOT a spotless SHELF QUEEN....
So ya' wanna clean'er up & make her pretty...
.simple is to take warm water with a few drops of liquid soap in it & a soft cloth to wipe them with.
A soft paint or shaving brush to dust them with first.  If you disassemble the cars be careful when bending the tabs holding them together.
Alternately you can use a commercial product such as Maguire's cleaner & polish to pretty them up with.
It's up to you as to keeping them original,  cleaning them up or completely restore them...

Happy Rails
dave