(Adds details from early trade, updates prices)
* TSX down 29.61 points, or 0.21 percent, to 13,985.53
* Six of the TSX's 10 main groups fall
TORONTO, June 21 (Reuters) - Canada's main stock index slipped on Tuesday as lower commodity prices weighed on energy and mining stocks, while industrials also fell after one of the country's major railways said it expected revenue to decline due to lower volumes and a major wildfire.
Canadian Pacific Railway Ltd slid 2.6 percent to C$158.85 after warning it expects revenue to fall about 12 percent in the second quarter from a year ago.
Rival Canadian National Railway Co was also among the most influential decliners, falling 1.8 percent to C$74.16.
The industrials group slipped 1.0 percent.
At 10:35 a.m. EDT (1435 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 29.61 points, or 0.21 percent, to 13,985.53.
Six of the index's 10 main groups were in negative territory, with decliners outnumbering advancers by 1.5-to-1.
The materials group, which includes precious and base metals miners and fertilizer companies, lost 1.5 percent as gold and copper prices fell.
First Quantum Minerals Ltd fell 3.5 percent to C$8.95 and Potash Corp lost 1.9 percent to C$21.21.
Oil prices also slipped, ending a two-day rally, with trading volatile ahead of a Thursday vote in Britain on whether to remain a part of the European Union.
The energy group retreated 0.5 percent.
The country's big banks and other financial stocks helped limit the index's fall, with Royal Bank of Canada edging up 0.3 percent to C$78.22 and Brookfield Asset Management Inc adding 0.8 percent to C$43.66.
Canada's federal government and its provinces agreed in principle on Monday to expand the national pension plan in a compromise that would see premiums raised moderately over time to provide greater payouts for pensioners. (Reporting by Alastair Sharp; Editing by Meredith Mazzilli)