two things hereFrom what I understand @anant_s to operate at the top speed of these TALGO carriages (200km/h) would require the existing WAPs to be running flat out and even operating at just below this speed (say 180km/h) would be incredibly taxing for these WAPs over prolonged periods thus the TALGO 250 trainsets are rather essential if the carriages are coming so as to utilise them fully.
From what you say, this is the very first step in a long road to get these trains operational as the trainsets themselves would have to come to trail in India next and then operate as a full train with the carriages.
- As of now only WAP5 can run at 150+ kph and as such there is no problem to run flat out at even 180-200 kph for it. Drivers though would require additional tests and training. Trivandrum Rajdhani runs a distance of ~500 kms at 130 kph without stoppage between Kota and Vadodra. Similar example for Duronto express are also present. So no issue with loco endurance.
- Distributed power is a universally accepted method for HST. Talgo trains are no different. Its 2 power cars provide about same power as a WAP7. but these owing to different bogie (meaning under carriage, wheel and suspension) design and unique tilting mechanism, allows the train to negotiate tighter curves at speed, without slowing down. if u recall ur mechanics lectures, objects moving on a curve have tendency to get thrown out tangentially. This is adjusted by either reducing the linear velocity or increasing the radius of curve. Now we dont want to compromise on speed, so next thing is make the curve gentler (increase radius), which again requires track modification, that we dont want (it adds to cost). Trains that can tilt in such a way that outer wheel is on a higher elevation, is another method of achieving the same and this is exactly what Spanish design is.
The train in trial could still have run with its power car, but that would have meant more time to allow meeting above requirements. By putting a proven locomotive, IR has bypassed the same and at the same time allowed speed trials at required speed to go ahead. This data can now allow IR to make decision including evaluation of safety aspects.