You don't put a fuse between the transformer and track because guessing what value of fuse to use would be pure guesswork. Too low a value and they would continually blow; too high and they would not protect your power pack or locomotive. Also, what do you mean by "old" transformer. Some may be too old for use with newer locomotives.
A type 1156 automotive taillight bulb is usually recommended to absorb excess current from short circuits.
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