do you mean in the immediate aftermath of ordering building of railways
most likely you are replenishing your stockpiles which are now below levels at which you start selling goods onto the world market, and thus have to wait until those are replenished to be able to export again.
Or are you talking after the rail are completed? If you are running a close inventory, you might be using more of your local production to maintain these new factories (factory efficiency does increase with rail, but so does, marginally, the amount of inputs used) which could cause a dip in exports if you are exporting less excess production of those factory inputs than before.