> Hybrid turbine/electric propulsion has been used on trains for decades…
I'd love to see a citation for this statement—it's far more accurate to say that turbine/electric propulsion has been tried for decades. Since 1969 there have only been a few test units built. [1]
The largest fleet of gas turbine-electric locomotives was operated by the Union Pacific until 1969. "Fuel economy was poor" and the units were not considered a success. [2] These units are quite well-known in railroad circles.
If gas turbines were as good as you say they'd be in heavy use on railroads today. Instead the dominant motive power is electric for lines already under catenary and diesel/electric everywhere else.
[1] http://en.wikipedia.org/wiki/Gas_turbine-electric_locomotive