The more wattage a transformer has, the more trains and accessories it can power. Track length has little to do with transformer wattage except that poor connections between tracks cause losses around the layout. Increased transformer voltage is needed to overcome these losses, and some wattage is lost in the form of heat at bad connections. Keeping track connections clean and tight minimizes losses and allows transformer power to go to the trains, and not overcome bad connections.
Having multiple lockons around the layout helps to distribute power evenly.
At 70 watts, your Marx transformer will operate one train and possibly a small accessory.
Make sure the power cord is in good shape, and change it if necessary. Install a 4 amp circuit breaker or fuse in line with the wire to the track to protect the transformer.
Larry