You are looking at the train indicator, an element unique to the map "Snowplow".
Snowplow page on the Official Team Fortress 2 Wikia:
Snowplow is an Attack/Defense map which was intended to be released alongside the End of the Line Update, but wasn't included, as Valve deemed it "too confusing and challenging for new players"
The basic idea behind the map is that one team has to get the train into the other teams base, while the other team has to place obstacles to damage the train.
The train symbol is an indicator to the trains current health.
The % symbol means "percent". It derives from the English words "per centum", which was eventually shortened to "percent". It pretty much means "per 100". If you are at 100% health, your health is full. If it is at 50%, you have half of your total health. At 0%, your health is at 0.
If you are on the BLU team, you are on the offensive. Your objective is to get the train into the enemies base. By capturing control points, you will remove obstacles placed on the track, and move the train closer to the RED base.
Each time the train hits an obstacle, a timer will start. When the timer hits 0, a missile will be shot at the train, reducing its health by 10%. The timer will reset and continue to count down, until the control point has been captured, or the train has been destroyed. When the control point is captured, the train heals 10% of its health, and continues to the next obstacle.
If you are on the RED team, you are on the defensive. Your objective is to prevent the train from getting into your base, by preventing the BLU team from capturing the control points along the line.
If you find the map to be confusing, this is likely expected. As the quote reads, the map was held back; The main reason being that it was not "newbie friendly". That said, these sort of things grow on you, with experience.
You will find a more in-depth analysis of the map here, along with a better explanation of the game rules - including a run down of all the control point locations, and the multiplier conditions for adding time on capture.