QUOTE (SirWilliam @ Nov 26 2017, 01:29 PM)
Does anyone know the sums on how much it slows a train down by the addition of an extra carriage ? This has always been the stock excuse when ever one enquires but I have never discovered by how much per additional carriage .
The days when you could "add and extra carriage" are long gone.
The railways now run fixed formation trains. Yes the HST's can be changed in size but this is not easily done as they run under the same "fixed formation" rules. Also no carriages exist to make them longer than 8 car.
The new Hitachi trains are total fixed formation and cannot be changed at all.
The main criteria that prevents longer trains is the length of platforms.
Many platforms are having to be extended to take the new Hitachi trains.
Also, in the modern railway "extra carriages" don't exist. The system is pared down to the minimum in order to "sweat the assets". Again, a factor determined by the DFT, not the TOC's.
In answer to your question though, 1 extra carriage on an HST makes little difference except when facing the steep banks in South Devon and Cornwall when it can be a problem. This is because HST's have fixed amount of power determined by the 2 power cars at each end.
Most fixed formation trains however have the power under some or all the carriages (like the turbos) so to add extra would also add extra power.