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Great... it seems that many people in Calcutta like Tata-Marcopolo Buses more than the Ashok Leyland ones in Non-AC CSTC travel. Similarly, many Calcuttans like the Volvo Buses more than the Ashok Leyland ones in AC CSTC travel.
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AC-12D (Howrah - Joka) travels from Park Street to Alipore Zoo (Near Taj Bengal) via Exide Xing Signal but during traffic jam it may divert its alignment via Victoria Memorial Back Side Road to come near Race Course-PTS Signal.
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Similarly, S-45 (Kolkata Station - Behala Sakuntala Park) travels from Alipore Zoo (Near Taj Bengal) to Majerhat via Ekbalpur (Kothari Hospital Road) but it may divert its alignment via Woodland Hospital - Lal Batti - Judges Court Road or via Woodland Hospital - Lal Batti - Alipore Road (Siddharth Building) Signal as per the bus staff's traffic analysis that too when it has no passenger bound for Ekbalpur/Mominpur/Burdwan Road.

I agree. I've traveled by some new smart CSTC buses under JNNURM Scheme and I've made the following observations:-
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Volvo AC Buses have broad seats made of thick velvet foam material with large standing area (CSTC doesn't run Non-AC Volvo Buses as of now). Routes like AC-1, AC-5 and AC-6 run Volvos.
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AshLey...
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AC (and Non-AC too) Buses have narrow seats with rag-zine foam material and narrow standing area. Routes like AC-12D and AC-20 run AshLeys.
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TataMarcopolo Non-AC Buses also have narrow seats with velvet foam material and narrow standing area. Routes like S-45 and S-47 run TataMarcopolos occasionally (Other times with AshLey Non-AC).
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PS: I haven't tried TataMarcopolo AC Bus travel since they are running on WBSTC Routes. I hope that WBSTC starts some AC and Non-AC Buses from Shalimar Station also some day :)