T1 leased lines offer speeds of 1.544 Mbit/s upstream and downstream. T1′s are used in America, Japan and South Korea.
Why 1.544Mbit/s? Well AT&T Long Lines was conducting tests in Chicago. They chose the bit-rate based on the speed that could be achieved over 2km long copper cables (once a safety margin was added). Why 2km? It's because the loading coils and cable vault manholes were 2km apart.
In Europe, E1's are used instead of T1's. E1's offer 2.048Mbit/s downstream and upstream.