The Principal Secretary in India denotes a government official of high seniority and is commonly found in the state governments. There is only one Principal Secretary to the Government of India

Government of India

The Government of India, officially known as the Union Government, and also known as the Central Government, was established by the Constitution of India, and is the governing authority of the union of 28 states and seven union territories, collectively called the Republic of India...

 and that is the Chairman of the Railway Board who is the ex officio Principal Secretary of the Ministry of Railways

Ministry of Railways (India)

The Ministry of Railways in India is in charge of the Indian Railways, an organisation that operates as a monopoly in rail transport in India. Dinesh Trivedi is the current minister responsible for railways....

. In the Indian state governments a Principal Secretary ranks above a secretary but below the rank of Chief Secretary

Chief Secretary

The Chief Secretary is the title of a senior civil servant in members of the Commonwealth of Nations, and, historically, in the British Empire. Prior to the dissolution of the colonies, the Chief Secretary was the second most important official in a colony of the British Empire after the...

. While each state has only a single Chief Secretary, some senior officials may hold the rank of Chief Secretary and are designated as Additional Chief Secretaries or Special Chief Secretaries. Principal Secretaries are commonly drawn from the Indian Administrative Service

Indian Administrative Service

The Indian Administrative Service is the administrative civil service of the Government of India. It is one of the three All India Services....

and hold the "above supertime scale" payscale. The equivalent position at the Government of India is Additional Secretary/Joint Secretary. Principal Secretaries are the administrative heads of the various departments of a state government such as Home, Finance, Revenue and Commerce etc.