Bridge Rectifier questions
The voltage rating of the rectifier isn't important (so long as it is higher than the supply voltage) but its current rating might be. The Digitrax Zephyr Xtra can put out 3 Amps, so for safety your rectifier should also be rated for 3A or higher. Another possible problem is the voltage drop of about 1.5V, which could reduce the top speed of your engines and might affect other DCC devices.     

Since your Digitrax Zephyr Xtra is powered by a separate 13.8V DC supply you could simply wire the ammeter into the power input lead, eliminating the need for a rectifier and reducing voltage drop to a minimum. The DCC controller will draw a small quiescent current. If this is unacceptable then you could use the meter's zero adjuster to null it out.