NEW DELHI - They say it takes a thief to catch a thief, but India's Delhi Metro has hired a monkey to frighten off other monkeys from boarding trains and upsetting passengers.

The langur monkey, trained since the age of three months, has been patrolling monkey-prone stations on a leash.

In June, a monkey boarded a train at the underground Chawri Bazaar station and reportedly scared passengers by scowling at them for three stops. It then alighted at Civil Lines station.

The Delhi Metro Rail Corporation says it hopes the new hire will avert a repeat of that episode.

"It started working about a month ago and since then we've not had a single incident," said Anuj Dayal, a metro spokesman.

The langur's keeper -- or langurwallah -- is being paid 6,900 rupees ($150) a month.

Langur monkeys are similarly employed around the grounds of parliament and some government buildings in New Delhi.