I've been using a puzzle analogy to talk about beginnings, ends, and now middles in plot planning. Just like a puzzle, the middle is often the hardest part when writing. The easy stuff is already done and the hardest pieces lay overwhelmingly in view, waiting to be connected. To keep your interest in the puzzle strong, narrow your focus to a specific section. Work on those trees in the shadow or that brown train car. Expand from there. Focus on the next complication that blocks the path to your main character's goal. Interesting stories must show struggles and how they are conquered, or how they alter the MC's route or his goal. What is at stake if they don't succeed? This creates tension and tension is exciting.
Sometimes we tire of looking for a certain puzzle piece and want to switch gears. If you are tired of writing a scene, work on something else. If you aren't feeling it, loving the scene, your reader's won't either. Come back later when you are fresh. In the meantime, what else excites you? Are you tired of putting train car after train car together? Break it up with a different setting or type of scene. Maybe it's time to throw in a little romance or mischief. Move the group to a different venue or introduce a new character. Just make sure that it connects somehow later on.
In writing Secrets of the King's Daughter, there is a middle part where the the people of Ishmael become converted to Nephite ways--except for princess Karlinah. Now no one wants to marry an unbeliever--no one except the worst possible candidate. It would be boring if I showed scene after scene of suitors rejecting the princess or her avoiding a certain someone. Middles especially need some variety along with the tension. Something new has to happen. The fun is putting something in that your readers won't expect.
No matter which part of your plot's puzzle you are working on, write those scenes that currently excite you the most. Other sections will generate new excitement for you as you see where and how they connect. Fill in the whole puzzle this way for as long as you can. Embrace your story. Go work on that exciting end, middle, or beginning. You need to have something to work from before getting specific help later, so go for it. Refinement comes later as you check the whole thing during revisions.