This isn't a very important feature of the game. You will be able to observe how the market changes, however this is optional. Try focusing all of your attention on expanding your railroad empire or even winning different types of auctions. You will be informed about these changes quite often. Be on a look out for newspaper covers. They will tell you about the more important changes. You won't have to find out any of this on your own. You will be dealing with types of informations. Some of the goods may become unwanted. As a result, you will start losing money if you decide to carry them anyway. The only choice here is to ignore such an important or modify an existing connection. Sadly, if you decide to choose the second option, you will probably have to leave this area for at least a few years. It's much better if you're informed that some goods are demanded by a larger number of civilians. The first thing do to here is to try to increase the number of resources that are being delivered to your cities. The easiest way to do this would be to build parallel tracks and send more trains to your existing factories. Alternatively you could search the map for untouched factories and add them to your grid, however for the most of the time this won't be possible. I think all of this is rather obvious, so you shouldn't have any other problems figuring out what exactly has to be done.
It would also be a good moment to say something about a possibility of taking part in other competitions. Mayors of cities will present you with a lot of opportunities. If you decide to participate, you will have to be the first railroad company to deliver much needed resources or goods. I'd like to warn you, though. You shouldn't try reaching cities that have been located at the opposite end of the map, mostly because you will be rewarded with only 100000 dollars. It's not enough, especially since you would have to create at least one expensive connections. Try searching for cities that have been located in the vicinity. Remember that you will only be able to earn some extra cash that way. You will have to focus on better connections that will provide with stable profits.
Take a closer look at the newspaper covers. Thanks to that, you will find out more about changes on the market.