English, alongside Hindi, is one of the two official languages of the central government.

The Constitution does not list any one language as India’s “national language”. We have 22 eighth schedule languages and 99 non- scheduled languages. Some languages are also included in the "classical language" category.

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1 of Article 343 (“Official language of the Union”) says “The official language of the Union shall be Hindi in Devanagari script”, and “The form of numerals to be used for the official purposes of the Union shall be the international form of Indian numerals.” English was initially an official language for 15 year period.

Section 3 of the Official Languages Act, 1963 provided for the “continuation of English Language for official purposes of the Union and for use in Parliament” even after the expiration of the 15-year period after the commencement of the Constitution.

So, English and Hindi are used on train boards as IR is under Central Government.

In various states, the official languages of the states are used in all services under the purview of the state government. Eg: Tamil in Tamil nadu, Malayalam in kerala, Marathi in Maharashtra, Punjabi in Punjab, ......

While agreeing with what you said I wish to say that there is nothing wrong in knowing another language. Apart from Tamil and English I can understand, speak and write Hindi to some level, can understand Malayalam and Telugu to some extent though I must admit not so familiarity with kannada. I know Spanish at very basic level.

Other Indian languages which I do not know but attracted my attention through media are Marathi, Bengali, Punjabi, etc
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I also read Sanskrit when we were in North according to my mother.

Outside india German, Japanese are among the top demanded languages for job prospects.