Hi All,
Apologies for mixing the in and outputs. How foolish. Also, I should have been more precise with the specs so here it is as it follows:
Amplitude = 12vp-p (+6V -6V )
Output current = 400mA
Adjustable from 1Hz to 1kHz
Output impedance 50 Ohms
Source voltage - can be anything I was hoping it could be done via 2 standard 9vdc batteries. But again, as long as it is reasonably portable I don't mind attaching it to any source

Tolerance of frequency is negligible.
Tolerance for duty cycle is +-5%.
Tolerance for voltage is not really good, basically, the output has to be 12Vp-p or slightly higher otherwise the equipment won't recognize the signal.
Unfortunately, the equipment under test is an ancient WSP kit on HST locos, and I haven't got a drawing for it (nobody seems to have it as far as I am aware). So all the above-mentioned specs have been measured by me. Using a multimeter in series to monitor the current, and adjusting the sig-gen to see when does it stops reacting to the input signals.
Back in the olden days when the equipment was new, they used to use an op-amp based test equipment powered by two 9Vdc batteries with an output voltage of 18Vdc, but it won't work anymore as it has been modded to an unknown extent.
I looked at all the suggested the methods, the microcontroller seems to be promising. I think the 555 won't be good because of the limited output current. The op-amp also sounds like a good solution however, I honestly can't get my head around the dual power supply thingy.
In fact, I attempted building that circuit twice by using two 9Vdc batteries and an lm741 but could not get it to work. I am so advanced, I can't even get my head around of how to limit the 18V source voltage to 12V.
Don't get me wrong, I can build just about anything provided I have a detailed drawing, it is just the lack of understanding which stops me most of the time.
Thanks again for the help and links.