It is very unlikely that clothing shops should be an oligopoly. An oligopoly is a market structure in which there are only a very few large firms competing with one another. Is this the case in India? In the US, at least,clothing stores are very numerous and would be in monopolistic competition. I don't know what data you have, but you should try to find out about the market share of various clothing shops. If there are only a few that have a large market share, then it is an oligopoly. Otherwise, it is not.
As for Indian Railways, it is a monopoly in that it has only a very few competitors, none of which is anywhere close to it in market share.
All retail shops are generally oligopolies, because of the limitations of customers to visit and buy from shops that are located very far from where they live or travel regularly. Clothing shops are likely to be even more oligopolistic due to the difference in the designs of the clothesthey stock and sell. In a country like USA or India, there are tens of thousands of clothing shops, but the shops that I can conveniently visit to buy a shirt will be limited to just about 100 shops in the city where I live. Further, the blue striped shirt of a particular design that fits me perfectly that I find in a particular shop may not be available in any other shop, or even if it is available, it may require a lot of time and effort on my pat to locate the shop that has it. In this way each shop is unique, and customers may are likely to buy their requirements of clothing regularly from just a few shops. In this way each of these shops behaves likes firms facing oligopolistic competition.
Comparison of Indian Railways with clothing shop is rather strange. Anyhow, Indian Railways is the only organization in India that is a allowed to operate railways on on almost all the routes in India. The only other organization, I know that runs railway service in India is Konkan Railways, which is also in effect a part of Indian railways. In this this way Indian Railways is a Monopoly. The only competition it faces is from substitute services like road and air transport, and to that extent it may be considered an oligopoly, particularly for goods transport services for which road transport is offering it a tough competition.