I believe the incident you're referring to is the Ludlow Massacre. The details match up: coal miners on strike, governor sends in the National Guard (who were also reinforced by locals and by agents of a "Detective Agency," who were by this time used as specialized strikebreakers, especially once the unpaid militia started getting sick of it and leaving), and they open fire on the strikers when the strikers move to engage them. A train is stopped between the two groups, saving lives.

There had already been clashes between mine operators and the miners, leading to some deaths. Tensions were already near breaking. At first, both sides thought the National Guard would ease tensions, but then they realized quickly that the National Guard were there to serve the interest of the mine owners.

The militia General Chase, who was an ophthalmologist in peacetime, had little idea how to handle a strike. When a crowd of women gathered to protest the jailing of Mary Harris "Mother" Jones, a labor organizer, he formed up his cavalry. As the crowd closed in, he panicked, and kicked a sixteen-year-old girl to the ground. The outraged crowd tore him from the horse. According to reports, he shouted, "Ride down the women!", and several women were hurt by the sabers and horses of the militia. Having the National Guard attack a square full of unarmed women lost the militia all sympathy among the strikers.

The situation continued to deteriorate, especially after a strikebreaker was found strung up, but the final match came when the miner camp's unofficial mayor, Louis Tikas, was brought in by the militia for questioning about reports of someone being held in the camp. Sensing the tension, the militia set up a machine gun. In response, the strikers got weapons and took defensive positions. Three explosions shook the air. An agitator for the mine company, Karl Linderfelt, who was also responsible for calling in the militia, claimed to have set them off to call for reinforcements. The explosions started the battle.

During the day, a freight train stopped in front of the National Guard emplacements, allowing panicked people in the tent city, who had been pinned down through the fighting, to flee.

It should be noted that the detectives hired, from the Baldwin-Felts Detective Agency, had a reputation for violently breaking strikes. The strikers knew something like this was very likely. Accounts differ about the source of the explosions that set off the fighting and also about who fired the first shot. By the end of the fighting, Tikas was dead, as were 21 people from the miner camp, 3 militiamen, and a very unlucky passerby. After being left in the sun for days, the bodies of the miners' dead were dumped into a concrete pit, called the "Death Pit."

The massacre set off a series of flare-ups and crackdowns in the area. More fighting occurred at several other mining camps. It's part of a longer history of strike violence in America that, though decreasing, lasted at least into the 1950's.

http://www.newyorker.com/arts/critics/books/2009/01/19/090119crbo_books_crain Article about the event. http://www.du.edu/ludlow/cfhist3.html An educational site about the event.

Edit: Since the train incident seems to be what you're interested in, I'll add that accounts vary about its role. Some accounts don't mention it at all. Some sources write that the train deliberately stopped where it did to allow the people being fired at to escape. Still others suggest that the people simply used the timely arrival of the train as an opportunity to escape.

Edit dos: I found this in the extremely biased report written by the United Mine Workers of America:

Mrs. M. H. Thomas was another of the women who was shot at by the murderers. She, with other women and children, escaped to a nearby ranch, where most of them were forced to sleep in filthy stable stalls to evade the exploding bullets from machine guns and high powered rifles. When they ran for shelter Mrs. Thomas was so close to death that a bullet clipped out a part of her hair, and around the feet of her two little children played the machine gun bullets.

A freight train that came down the track about noon Tuesday enabled this party of refugees to escape. Knowing that the train would be between the gunmen and her people, Mrs. Thomas ran to the well and told others to try to make their escape. The entire party got away, but it was only because of poor marksmanship on the part of the gunmen, who riddled the air about them with hundreds of bullets.

Source: http://historymatters.gmu.edu/d/5736/

I also found this in a report to Congress. The fact that the militia had to use force to get them to move the train suggests there was intention behind where they stopped:

At 7 o'clock in the evening [note the wide discrepancy in times], a freight train pulled into the station between the pump house and the soldiers, who were directing a heavy fire into the tent colony. The women and children took advantage of the shelter its steel coke cars afforded to climb from the well and make a dash for shelter in an arroyo. The conductor and brakeman of the freight train testified at the inquest that they saw about fifteen women and children, crying and whimpering, scurrying along a fence near the railroad track. The trainmen testified that twelve militiamen covered the engineer with revolvers and ordered him to pull his train out and do it "damn quick," or they would shoot him. The engineer obeyed, although he had orders to take a side track at Ludlow in order to let a passenger train pass.

https://archive.org/details/reportoncolorado00unit