Union Pacific Corporation UNP is a stock that investors would do better to discard from their portfolios at the moment.The negative sentiment surrounding the stock is evident from the Zacks Consensus Estimate for fourth-quarter 2018 earnings (scheduled to be released on Jan 24) being revised nearly 1% downward in the last 60 days. The same for full-year earnings has been trimmed by 3 cents. The company also has a VGM Score of D. Here V stands for Value, G for Growth and M for Momentum, and the score is a weighted combination of all three scores.Let’s delve into the factors responsible for the stock’s dismal performance.Union Pacific’s high operating expenses are concerning. Operating costs have increased 8% in the first nine months of 2018, mainly due to a 41% rise in fuel-related expenses. The high operating expenses have potential to hamper bottom-line growth going forward.Additionally, declining coal volumes due to pricing pressures might hurt the company’s fourth-quarter results. At the Credit Suisse 6th Annual Industrials Conference held last November, the company stated that fourth-quarter volumes at the energy and agricultural segments dropped 10% and 4%, respectively, as of Nov 25.Union Pacific Corporation Price and Consensus Union Pacific Corporation Price and Consensus | Union Pacific Corporation QuoteThe company’s high debt levels are another downside. Debt/EBITDA ratio (adjusted) at Union Pacific is anticipated to increase to up to 2.7 in 2020. The ratio was 1.9 in 2017. A high Debt/EBITDA ratio often indicates that a company might be unable to service its debt appropriately.In terms of price-to-book ratio, which is often used to value railroads, the stock's valuation appears to be unfavorable compared with the market at large. The company currently has a trailing 12-month P/B ratio of 4.9 compared with the industry’s average of 4.3.Due to these headwinds, shares of the company have shed more than 2% of value in the past six months.The company carries a Zacks Rank #4 (Sell).Key PicksSome better-ranked stocks in the broader Transportation sector are United Continental Holdings UAL, Spirit Airlines SAVE and Azul AZUL. While United Continental holds a Zacks Rank #2 (Buy), Spirit Airlines and Azul flaunt a Zacks Rank #1 (Strong Buy). You can see the complete list of today’s Zacks #1 Rank stocks here.Shares of United Continental, Spirit Airlines and Azul have rallied more than 24%, 29% and 16%, respectively, in 2018.More Stock News: This Is Bigger than the iPhone!It could become the mother of all technological revolutions. Apple sold a mere 1 billion iPhones in 10 years but a new breakthrough is expected to generate more than 27 billion devices in just 3 years, creating a $1.7 trillion market.Zacks has just released a Special Report that spotlights this fast-emerging phenomenon and 6 tickers for taking advantage of it. If you don't buy now, you may kick yourself in 2020.Click here for the 6 trades >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Spirit Airlines, Inc. (SAVE): Free Stock Analysis Report United Continental Holdings, Inc. (UAL): Free Stock Analysis Report Union Pacific Corporation (UNP): Free Stock Analysis Report AZUL SA (AZUL): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research