Lightweight Non-Electric Loco

I am not an expert on trains and locomotives, and I don't know all of those that are out there. So if this seems a sillliy question, it is honest.

What is the lightest-weight locomotive out there for pulling passenger cars? I am talking non-electric, so the HHP-8 is out. Is there an alternative to the Genesis that may not be so large, noisy, or thirsty, particularly for smaller consists of only a few cars?

Theoretically you could take an old GE 44-tonner and convert one of the two engines to provide HEP for the passenger cars. That'd give you a very lightweight passenger loco. Not bad as long as you don't need to go faster than 10 MPH or so.

In terms of something that would actually be practical, perhaps something along the lines of a GP15T or GP15-1 with a separate HEP genset tucked inside. That would be perfectly adequate for pulling three typical passenger cars, and would weigh less (and use much less fuel) than an F40PH or a Genesis.

Vermont Rail System had basically the same setup for the commuter train they were running a few years ago, except they used a GP38. I mention the GP15 series because it is lighter and more fuel efficient than the GP38, per your requirements.

Heve a look at Talgo.
http://www.talgo.com/htm/English/productos1.htm