UPDATE 1-Brazil's Usiminas posts bigger-than-expected Q3 loss

* Loss at 124.9 mln reais comes above poll estimate

* EBITDA misses forecast on one-time railway expense

* Margins slump as mining, steel units underperform

SAO PAULO, Nov 1 (Reuters) - Usinas Siderúrgicas de Minas Gerais SA posted on Thursday a bigger-than-expected loss for the third quarter as weak iron ore sales and surging one-time expenses weighed down results at Brazil's largest maker of flat steel products.

Usiminas, as the Belo Horizonte, Brazil-based company is known, lost a net 124.85 million reais ($62 million) in the third quarter, wider than the expected shortfall of 91.5 million reais in a Reuters poll of 11 analysts. Usiminas had lost a net 86.51 million reais in the second quarter.

The company had earned 154.03 million reais in net income a year earlier.

Net revenue and costs rose at the same 5.1 percent pace on a quarter-on-quarter basis, indicating that the impact of selling more products in Brazil, where flat steel is sold at a premium relative to imports, was offset by higher costs stemming from a higher output of value-added products.

Net revenue, which totaled 3.389 billion reais in the quarter, beat the 3.250 billion reais forecast in the poll.

Sales, general and administrative expenses rose about 10 percent on a sequential basis, driving losses wider at Usiminas, the company said in a securities filing. One-time items including an increase in transportation contract costs and provision for some contingencies more than doubled.

Earnings before interest, taxes, depreciation and amortization, a gauge of operational profitability known as EBITDA, plummeted 36 percent as performance in the mining and steel units was disappointing. EBITDA totaled 150 million reais in the period, below the 213.6 million real estimate in the poll.

EBITDA at the mining unit fell to 36 percent of revenue in the third quarter from 47 percent in the prior three months. The steel unit saw its EBITDA margin, as the indicator is known, down to 2 percent from 6 percent in the second quarter.