according to the map it seems line 6 also connect to Sadeqiyeh station in Tehran . just like line 1.
its strange to me they could connect it to mehrabad airport metro that is just a little south of sadeqiyeh station . after all mehr-abad also provide service to People of Karaj
Where does the map suggest it's heading to Sadeqie? We don't know, it could be meant to end anywhere in Tehran.
Thing is, if extended to Tehran Line 6 of Karaj Metro will have to run parallel and close to the Tehran-Karaj commuter line, because the zone where it leaves Karaj is kind of a bottleneck: bordering the Alborz mountains to the north and further south is a fairly large uninhabited area comprising sand processing and concrete plants, so no point leading the line through there.
When it passes those five or six kilometers and enters Tehran, up until Tehrānsar almost everything south of Jāddeye Makhsuse Karaj (Lashgari Expressway) consists of industries and factories, residential neighborhoods being few and far between. It could traverse this zone nonetheless, after stopping at the densely populated Qods suburb let's say, and go on until e.g. Mehrābād. Another option would be to have it conclude its route at Vardāvard station of the Tehran-Karaj express rail, where Tehran's Metro Line 10 (currently under construction) will also have its terminus. A third possibility, which is not mutually exclusive with the previous one, would be to extend it to the central districts of Tehran in between Lines 5 and 10 i.e. south of Chitgar Lake. Its terminus could then be at a station shared with Tehran Metro Line 4 or 6 or both.
When it comes to the Mehrābād domestic airport, two aspects need to be factored in though:
1) Sonner or later, all domestic flights will be redirected to the Irānshahr Terminal (Terminal 3) of Imam Khomeini International Airport, construction of which should have gradually begun as we speak. Chances are that when Line 6 of Karaj Metro is inaugurated, Mehrābād will already have turned into an exclusively military airport. Thus it wouldn't make sense for Karaj's Line 6 to serve Mehrābād.
2) Considering that on a given distance, metro lines stop more often than commuter rails, riding the former will increase travel durations. Moreover Tehran's Line 5 / Karaj's Line 1 is an express train, running at higher speed than the metros. In other terms, even if they'd need to change trains thrice, passengers from Karaj would be better off using the express train than a metro. After all the distance between Mehrābād and the city limits of Karaj is no less than 25 kilometers.
The question of travel times by the way is a more general issue Tehran's rapid transit system will have to address. To me it should even be made a priority, considering that the inner city network is now developed enough. Tehran's surface area of 770 square kilometers is large enough to make metro trips quite lengthy when the route spans opposite points of the city. With a stop every kilometer on average, and possible train changes in between, it would take well over an hour from one end of Tehran to the other and more yet if traveling from the western parts of districts 21 or 22 to a destination on the eastern or southern edge of town.
The solution to this is to have several suburban express lines (between two and preferably four) traverse the entire city at fast speeds and with fewer stops, all of which would be interchange stations with one or better, several metro lines. Meaning that said express lines would arrive from some suburb, run through Tehran, exit the city and reach some other suburb on the opposite side of the agglomeration. Outside city limits these trains for the most part would operate above ground and underground within Tehran itself. Such a system is in place in various major urban centers, including Paris.
Map of the RER (express commuter rail) network in Ile-de-France (region around Paris). The RER is operated by the national railway company SNCF by the way, Lines A and B jointly with the RATP (Paris inner city public transport operator, managing the metro, buses and tramways) by the way.
As you can see there are five RER lines (line E is still to be completed after extension to the western suburbs). Most branch out in different directions at both ends. Except for the RER C in western Paris, these lines stop at very few stations in the city proper.
Typical RER stations in downtown Paris:
An overview of the interconnetdness between RER and Metro networks:
Now the urban rail master-plan for Tehran is calling for express lines to follow a similar pattern indeed (on an anecdotal note, a French company by the name of Systra has been involved in designing the network map). This is why at the current terminus of Sādeqie, you can see how rails on Line 5 enter into a tunnel. For after Sādeqie, Line 5 is planned to take an arc-like route to the north, followed by a short west-east stretch and a straight southward plunge all the way to Rāh Āhan (Tehran Railway Station). It would seem that Line 5 will mark merely two stops between Sādeqie and Rāh Āhan, namely interchange stations with the future metro Line 8 as well as with Line 3 at Enqelāb Avenue in the heart of the city.
In the previous master-plan, Line 5 express rail was extending further, from Rāh Āhan in a more or less right-angled change of direction towards the east until Se-Rāheye Afsarie in the Khāvarān area of southeast Tehran, before leaving the city along the Emām Rezā Highway to serve all corresponding suburbs and end at Pākdasht. This section was abandoned in the latest master plan worked out in 1397. Here's hoping that the project will be revived in future. Another change introduced by the current plan is that the number of express lines was lowered from four to two. On this as well, I'd tend to be more favorable to the former approach. Trams however were increased from three to five lines (more about this later).