Solders traditionally contained lead. Lead eliminates the growth of tin whiskers; which can short out electronic circuits. Solder containing lead also melts at a much lower temperature than pure tin solder. Lead is a hazardous substance which countries around the world are trying to eliminate. Europe, China, and the United States all have initiatives that are in effect or will be. Most companies are going to pure tin finishes, though other finishes like gold and Nickel/Palladium/Gold flash are also being implemented. What does this mean to you? What are your options?
Because of tin whiskers, military and other high reliability industries like medical, are exempt from the pure tin requirements. I work in the military industry. So most of what I can tell you is because of this experience.
DCC electronics that you buy today, regardless of what country you live it, probably contain electronic components that have pure tin finishes. Components with pure tin finishes can grow tin whiskers. There isn't anything you can do about this, but if you want to learn about tin whiskers go to my advanced section on the topic.
As far as soldering decoder wires in locomotives and wires to track and buses, you have a few choices. You can use pure tin solder, continue using solder with at least 3% lead, use solders called "SAC" solders, or in the case of attaching wires to buses, using insulation displacement connectors (IDCs). Because pure tin melts at a significantly higher temperature, pure tin solder is not common.
The solder selection guide shows some of the more common solders available. Pure tin clearly melts at a much higher temperature than tin/lead solder or SAC solder. In most cases, using pure tin solder will simply require more heat. If you have a temperature controlled soldering irons station, you will need to dial it up. Using pure tin solder probably won't be a big deal except when soldering to track where a higher temperature and more heat may mean melting ties. It could also be a problem when soldering to electronics due to the higher temperature required. Therefore, you will want to use something like SAC solder or continue using solder with lead.
If you continue to use solder with lead in it, you need to treat the solder as you would any other hazardous chemical in your home. If you have children, you should keep the solder locked up and out of their reach. Wear disposable gloves when using this solder. You should not eat anything while using this solder and should wash your hands thoroughly when done. In Europe, it may be difficult, if not impossible, to buy solder anymore with lead in it. For those in the US, you can still buy leaded solder. If you want to take advantage of its lower melting point, you may want to buy yourself a lifetime supply.
There are a multitude of SAC solders available. SAC 305 is probably the most common. It contains 3% silver and 0.5% copper. Just in case you are wondering, SAC means Sn (tin), Ag (Silver), and Cu (Copper) per the periodic table.