This question has been debated time and time again in the news media networks with savants speaking for both sides, but from the most commonly used rhetorical perspective of “Because the rest of India uses broad gauge…”
Let us delve a little deeper taking references from other train systems (not high-speed rail) from around the globe to understand if there are any benefits to retaining similar standards and trains, when building new systems.
Interestingly, most older metro systems have inter-compatible trains that run on similar gauges. Older systems like New York and London have inter-compatible train systems that run on similar tracks (gauges) even though they were combined by separate companies. It must be noted that these train carriages might be different, but the track gauges used were the same. Older lines were cheaper to build back in the day when it came to tracks or acquiring trains, and hence capacity was not much of an issue. The solution to most problems were conveniently solved by throwing more money at it without much thought about efficiency. They would order larger trains to solve capacity problems rather than more efficient usage of trains with higher frequencies. Hence, older systems tend to have larger trains.
Modern lines tend to have different train types and technologies across different lines, as new lines are contracted as tenders. So, the regulator (Government) would often package the whole line as a single unit so that companies could competitively bid for them. This increases diversification as different operators have different models and tended to work with their own partners. Further, newer systems have modern signaling that allows faster frequencies and hence uses smaller trains. Newer systems like Singapore, Shanghai, Taipei and Hong Kong tend to use different train types and technologies across systems, and nobody worth their salt would complain for the service levels of these transit giants.
Another perspective pondered, is if it would be beneficial for maintenance works as there would be common parts and cross utilization of mechanics and other resources? But, this is a myth. Speaking in terms of railway engineering, when it comes to maintenance, equipment is usually similar across different track gauges and train sizes. Even seats are standardized across train sizes.
So, to allow the use of dedicated maintenance equipment for a particular system/line really depends on the fleet size of the line and if it attains a critical mass. Moderately long lines need dedicated facilities. Short lines (3-4 stations long) like the U4 in Berlin or the Yellow Line in Montreal are connected to a main line so that they can utilize maintenance facilities and rolling stock storage.
Interestingly, cities like London, Toronto, San Francisco and Beijing are building new lines based on international standards moving away from local standards. Standard gauges are the modern way to go. It helps international companies compete for trains and thus elevates the systems.
Lastly, independent lines make for more resilient networks and hence system cross reliance is not always a good thing.
I have laid out all the arguments that a typical railway engineer would tell you when asked the same question, but as a transport policy economist, I would go one step further to state:
The choice of using similar standards across your systems does not give us any significant advantages, or at least not in the way everyone seems to think. That being said, we cannot exclude the fact that there are cities who use similar standards across systems and still give commendable service. So, context is everything, and the right assessment with the right information would be key to any sort of conclusion in the matter.
Dr Sreyus Palliyani is a Transportation policy expert, who specializes in public transport benchmarking and best practices for policy and governance.