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Tony,I have not built such a system, but it follows some tried and true princples that I've used on many other systems for years.  Right now I'm using a variation of it to regulate the voltage for my tortoise switch machines for my indoor HO layout.  Simplicity is the key.  Here's what you need:Full-wave bridge rectifier - Takes power from the rails and corrects the polarity no matter which direction you place the locomotive on the rails, or even if You have AC running on the rails.  It costs under $2, is a black platic square with 4 leads on it.Adjustable Voltage Regulator - Feeds the batteries the precise voltage they need for float charging.  Float charging is the voltage that you can keep the batteries at forever without boiling them dry.  For lead-acid batteries it's 2.25 volts per cell, or 13.5 volts for a 12V battery.  They're self current limiting.  You want to limit the current to approximately double the current draw of the locomotive.  For example, if the locomotive draws 1.5 amps, you want a 3 amp limit.  This is not a hard and fast rule, but gives you a starting point.  You may need to add a power transistor to give the regulator a boost in capacity.  You'll need to mount them both on a heat sink, as it'll dissipate 18 watts of heat worst case.  The last regulator I bought cost $0.50 and had a 1/2 amp output.  A power transistor to boost it up would cost around $2.  A few resistors are pennies apiece.  The heat sink is the big ticket item, for around $5.Power Supply - For a 13.5 VDC battery system, you need 13.5 + 1.2 (rectifier drop) + 3 (regulator drop) + 2 or 3 for voltage drop on the rails.  This puts you at a 19.7 to a 20.7 VDC supply.  The only problem you get from a higher voltage supply is more heat to be disipated by the on-board voltage regulator for the batteries.  You also need to have enough "poop" to feed the maximum draw of the locomotive, which in this example would be at least 3 amps.  You can also use an AC supply, which should be in this case 20.7 / Sqrt(2) = 14 VAC.As for maintaining track power, you can pick and choose which sections of track you want to insist have power, and which sections of track you'll allow to be hit or miss.  If you only want to make sure you have power in the staging yard, so be it.  The more track that has power to it, the more run time you'll have before the batteries run down.  You can make up for it some by instead of doubling the output of the regulator, tripling it. As for cost, using discount parts I could cobble a system together for a locomotive for around $10 and would be the size of a deck of cards.  While this may cramp the space in a locomotive, you could make up for some space by using smaller batteries.I'll have to dig around a bit to get some possible part numbers and such if anyone's interested.  To be honest, I'm surprised that it's not standard issue for a garden railroad.Mark in Utah
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