Perhaps the chosen ETR1000-based trains might be reconfigurable, depending on what their onboard control systems can handle. The design's base 8 car configuration has 4 power and 4 trailer cars in formation DM1-TT2-M3-T4-T5-M6-TT7-DM8, so a notional extension might add cars in pairs of 1 motor, 1 trailer, without changing the power-to-weight relationship and thus maintaining performance, if other onboard systems can be reengineered to support that.Would the 895s have a HV bus, like the 390s? If so, could the new design allow reconfiguration in the future? Just as some of the 390s were extended from 9 to 11 cars, an ability to do similar might allow creating a balanced fleet of 8, 11/12 and 16 car units from an initial all 11/12 car fleet. After all, many 3rd rail classes have been varied in length during their working life, as have the class 22x DEMUs.
However, HS2 really shot itself in the foot with the following clause from the rolling stock specification:
So they may have been offered and could be contracted to purchase a train that cannot easily be reformed, either in manufacture or subsequently.5.2 The nominal length of the Unit shall be 200m. It has been determined that all Units should
be the same length, with no requirement for a Unit to be capable of being lengthened or
shortened. This supports the overall HS2 business case.