On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 13 - Locomotion. Our step-by-step walkthrough explains how to find all (3) secrets hidden in the 13th chapter of the main campaign.
The first treasure is already at the beginning of the level, inside a railcar. To get it, you have to jump into the railcar through the window (don't climb the ladder to get on the roof). The treasure lies at the end of the train.
The second treasure is on the roof of a railroad car. In this train wagon, you will be attacked by an opponent (scripted sequence) - he will jump in through the window and attack you.
The last secret is inside the cargo wagon, just behind the one with the coal (the only one during the whole stage, so it is rather difficult to miss it). To get inside it, you have to stick to the left edge and when you get to the door, jump on the footbridge (you shouldn't have any difficulty finding the entrance).