trains uckfield

Why is it that when the trains do not use diesel electric truck, even if you could?
If the trains are diesel / Electric then surely could have a pick-up to receive electricity through third rail, where I live there is a railway line, half of the diesel form, but the other half is electric (binds to a power), why this train does not use both sources? (It's the Uckfield – London Bridge Line). I saw the French, the Dutch roads, iron German and Czech. Without doubt, the cost of diesel is higher than electricity? Well, I mean a train using diesel as fuel, although on a track with electric pickup available and the train is a diesel or an electric one, so it uses the electric current, even when apparently trains railway line near me, are "Turbostar"
could be a number of reasons for diesels operating in the diesel supply line. first question is how far the train leave the electrified territory? it may be easier for railroads to operate the diesel all the way, despite having access to some railroad wire/3rd. Some are equipped with diesel engines (at least here in the U.S.) with a combination of equipment, which means that function as a pure fuel where no cable or 3 rail, and once they enter the territory electrical switch can operate as an electric. The main reason for this is to reduce engine emissions in urban areas congested.
Buffer Ruffer 6.1 Railtour At Uckfield