Well, my explanation was for a freight train only , and ideally, cannot be extended to passenger rakes just like that. The main advantage with python rakes is that in most cases there is absolutely no need for any shunting operations. The front portion detaches and goes on it's wy while the rear portion continues on it's route. This can be even done with just a single long loop line, without blocking the mainline.
Passenger rakes will need much more, as they will not be run with a loco sitting idle in the middle. So they have to be shunted, and this will require a larger yard. I can think of the following problems with your proposal....
more...

1) Yard construction and maintenance : Most large stations like HWH etc have the pit lines just before the stations at the outer. Now, the outers are always very congested, with trains always being made to wait for clearance. If 8-9 coaches for multiple trains have to be shunted from the pit lines to some yard far away from the station, they will add more to the congestion by blocking the main lines. So, the only option is to have pit lines and maintenance right at the site of the new yard so that rakes need not be moved for long distances.
But the problem is that pit lines and yards require a lot of space, infrastructure and personnel. There isn't anything like a simple passenger yard. They would need all the equipment, pit lines, washing lines, sick lines etc just like any other coaching yard. This requires a lot of space. And space is something that most of our cities don't have anymore. And there needs to be a minimum amount of traffic before the construction of a yard becomes useful.

2) Logistics : Even if a yard like that materializes, then there is the issue of safety and managing operations. The train will have to enter the yard at a reduced speed, wait for 5-10 minutes for the shunting operation, and a few minutes for doing the brake testing for the combined rake, and again waiting for clearance on the main line before proceeding out. This will eat up close to 20-30 minutes in most situations. And I am sure passengers will not appreciate their train being shunted about in some random yard for half an hour, just after their journey starts.
Managing tickets and UR passengers will also be a problem. As the extra coaches are...
more...

being attached in a random yard,only the subsequent stations can utilize these coaches. So reservations etc will have to be managed. And in the return journey, they have sped some time at the penultimate station to make sure that all the coaches which have to be detached in the yard are empty and that there is no one left behind in these coaches. This will only result in accumulating delay, safety problems and mainly lots of headache for the passengers and the railway staff. It's a good idea, but there are so many constraints in implementing it that it ends up giving more problems than utility.
Freights are just loaded once at their source and unloaded at their destination. So they need not worry about all these operations and the job is much simpler for them. But for passenger rakes there are many things that have to be taken care of before the train can operate.

Now let me tell u the operating reason apart from these why coaches cant simply be coupled /decoupled right in the middle of the journey.. Let for our case assume that the stn choosen for attach/detach in HBC chordline is BRPA approx 24 km from HWH & 71 km frm BWN.let say a 15 coach train starts it journey at 7.10 frm HWH crosses DKAE at 7.29, JNX at 7.34 to reach BRPA at 7.41. In this process it overtakes a freight at DKAE & JNX which waited for 19 mts each at respective stns to get overtaken & commence their journey at 7.37 & 7.44 respextively. Our Exp after addition of Slip coaches restarts its operation at 8.05 frm BRPA but by the mean time both the freights have moven further forward & again they need to be looped at CDAE at 8.17 & BMAE 8.23 for reovertake by the...
more...

same Exp. So this is nothing but killing precious time of other trains. So u can see even though the slip coach attachment /shunting going on in the yard & doesnt affect anything on the mainline still the running in mainline is hampered justt because the train waited an extra time in the yard. This is prexisely the case with ur pythons also.. Apply the same logicslightly change the factors & you can understand how coupling/decoupling is for a long run not useful for a section & doesnt solve any purpose regarding operating point of view. Regarding commecial viewpoint they obviously solve a huge purpose. Just like our 15 coach imaginary train it has such mad rush that pax wont mind if they get berths from 26 km advance locn, tkt sales would rise & provide extra revenue to Rlys. Because commercialy it is always viable to run one 24 coach train than running two trains one with 15.other with 9 coaches. But the operating norms will simply dont allow such thing to happen.

That is exactly what I have been teling about Python rakes. They are not used for short runs. Python rakes are typically used for long stretches of 200-400 kms. And in addition do not jumble up the operations of a python rake with that of a passenger train with slip coaches. Both are completely different scenarios. A passenger train with extra coaches is still a single train. But a Python rake is essentially two 'separate' trains.
And no, there is no time wasted for a python rake in the yards. This is how it works.
...
more...

For example, consider 2 freight trains going from Jolarpet to Chennai, a distance of about 220 km. Now, JTJ is an interchange point between SWR and SR. So all freights enter the yard as they need to be inspected by the Directorate of Wagon interchange ( DWI). Basically, freight trains pool wagons from multiple railway zones to ensure maximum wagon utilization. So to ensure that no particular zone is being forced to pool more more wagons than the others or vice-versa, at zonal and sometimes divisional interchange points, freights are inspected, with the staff making a note of each zone's contribution in the rake, the health of the wagon etc. This can take anywhere from 2-4 hours for a fully loaded freight train depending on the number of freights in the yard and the traffic on the mainline. This is a mandatory halt time and it cannot be avoided.
Now in case of our example at JTJ, a typical journey from JTJ to Chennai for a freight can take about 6-10 hours ( SR's average freight moving speed is about 23 kmph). And during the day time, there are two open slots between 11:30 - 13:20 and 14:00 - 16:45, which have very less passenger traffic from JTJ ( this does not include any freights already on the route and the traffic between AJJ-MAS and KPD-MAS). For the 2 freights which moved into JTJ at about 9 AM and 10 AM in the morning, the inspections would be complete by 11:30 AM and 12:30 AM respectively. Now, with both trains departing separately, they would only reach Chennai by evening or in the night, and they would require a crew change en-route if they get delayed. And in addition, there are multiple freights waiting in the JTJ yard which are bound to other destinations like BZA, RU, KPD-VM etc. So, since both freights have the same destination or they at least have a common route till Chennai, it saves a lot of time for both trains if they are coupled together and finish the journey as one rake.
So, the train starting at 11:30, may be delayed by one hour to start at 12:30, but instead of reaching at 6 PM or later, it will reach by 3:30 PM and in addition, the 11:30 slot may be used for another freight and the tracks can be utilized better instead of keeping them free. And the additional benefit is that two freight trains use the same slot to reach Chennai.
Now, even if one of the 2 freights has to proceed further from Chennai, it is no delayed due to the python rake formation. This is because, Korukkupet/Tondiarpet yard is the last yard in SR before the trains enter SCR and most trains get the wagon regulation checks done here.
All this works because freight trains are not like passenger trains. They have long halts at many stations, unlike passenger trains which have short 5 minute halts. Most of the times, we can see multiple freight trains being made to wait at the wayside stations. These freights usually halt for at least a couple of hours before leaving. It is quite simple to see that, when two trains are anyway being made to wait for 2 hours at a particular station, it would save time if both are coupled and leave together ( of course, only if the traffic on the route and the availability of loop lines at intermediate stations etc is also suitable) instead of making them leave separately and occupy two slots.
Sorry for the long answer. :)