As an environmental scientist--
Natural gas is "clean", contingent on the method of extraction. For example, fracking is NOT a clean method of obtaining natural gas. In many cases, the process releases a large amount methane (a GHG) into our atmosphere--not from burning the fuel, just because of the extraction technique. On a molecular basis, methane is a MORE POTENT GHG than CO2. When people argue against natural gas, the argument is:
(1) It's NOT a renewable fuel, which we need, for many reasons.
(2) Extraction methods aren't the cleanest, at which point natural gas isn't actually better than, say, coal...which pollutes more in burning, but is "cleaner" to extract, and is more CO2 heavy than Methane (CH4) heavy...which is significant.
In an ideal world, natural gas extraction wouldn't be done via fracking, which would cost more, but would also actually make natural gas a relatively clean option. As for being an "enviornmental wacko" (or what have you), an amazing amount of evidence now exists that demonstrates humans are causing global warming and climatic changes. For instance, right now, there is a very strong hurricane in the Atlantic (Mathew), which I expect to be international news for the next several days because it's going to absolutely destroy Haiti, the Bahamas, E. Florida, the S. coast of the US, and possibly the NE US (Stay tuned there because some of the guidance would shut down SEPTA, NJ Transit...etc, for several days but it's too early to tell for certain unfortunately). And, unsurprisingly, all of this is happening because the sea surface temperatures in the atlantic are several degrees warmer than they should be at this time of the year, fueling this hurricane. To the point: environmentalists are lobbying for clean energy because we NEED clean energy at this point to slow the effects of climate change. Whether that's the paris agreement or an individual power plant, it all matters.
With the above being said, I will say that natural gas is apples to apples cleaner than coal if done right. On the flip side, one of the benefits of public transit (in general) is that it's cleaner than using a personal vehicle for transportation--whether it's a bus or a train, especially trains, which are the cleanest transportation option available. The method in which we power said buses and trains is however also important. If we're using a coal (or fracking-sourced) natural gas, power plant, to fuel trains and buses, it's not actually a "clean" solution. Electricity is only as clean as the method used to generate it.
I also disagree that we cannot power a train line (or anything else), with renewable energy. It is entirely doable. The question is whether or not the investment is made in it. The argument of "when the wind isn't blowing", is not valid, because for the most part...wind farms aren't built where the wind is often not blowing! That'd be pretty stupid. Moreover, wind farms are designed to maximize the amount of time in which the wind *is* blowing, and adjust resistance to ensure that the turbines are spinning a majority of the time. Similarly, solar power is at this point very reliable, there's hydroelectric (has issues but is still clean), etc. The point is, environmentalists are advocating (as they should) for a society which is less dependent on fossil fuels. It's safer for the US (reduces reliance on questionable international sourcing), and it helps the environment. Considering we're talking about public transit, it makes sense to be thinking about clean energy options.
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