BEIJING—Erin Jackson of the United States won gold in the women’s 500 meters speed skating with an explosive start on Sunday, giving the United States its first gold medal in the event since 1994 and first speed skating medal at the Beijing Olympics. She could be seen grinning widely after completing her lap of the National Speed Skating Olympics in 37.04 seconds, but had to wait for the last pair to finish before embracing her coach and bursting into tears. “I wish I could describe how I feel. It is amazing. This medal means so much,” an emotional Jackson told reporters. “It has been a tough couple of years and a tough beginning for this year. For this to come around like this I am so happy.” Jackson could have missed the opportunity to represent the United States after a stumble during the Olympic trials, but her Brittany Bowe gave …