By “nowhere near the city”, do you mean that there are no town-buildings for the entire length of the segment? How long is the road-segment? If it’s a very long segment, it’s possible to be both nowhere near the city and in the city at the same time.
If it is near a city, it might be the case that the widening of the road could be blocked by a building so it won’t be possible to make it wider. Also, The widening of a road can be blocked by other infrastructure such as industries, stations, depots, bridges, railways, or even other roads.
This is one of those many niggles you’ll encounter when playing train-fever. You can learn to work your way round them by trial and error and build up you own set of ‘rules of thumb’. In the meantime, you can split your road-segment in half (build a road-junction about halfway along the segment), and try upgrading both halves. For any half that does not upgrade, split it into half again and try again. Eventually, you’ll see what’s preventing your road from being upgraded.