I found that when the player narrowed down with the schedule GUI with this button, it was inadvertently changed to an incompatible line and the schedule changed, so as a countermeasure for this I changed to lock the button of the incompatible category.
That is, a passenger-only convoy can only choose whether or not to narrow it down. But I think this change will also improve clarity. For example, you can prevent accidentally changing a passenger-only convoy from a passenger/mail mixed line to a mail line.

@James - Please check pull request #201.
The had inspired me to have a full combobox for the line window in standard specifying freight as well as the exact type of freight in r9140.
I think it is reasonable that standard chose full combo box. Comparing statistics is more convenient if player can narrow down to the same category line.
I initially narrowed down these three categories as broad categories. This is for future expansion. I didn't use any special symbols or extended specifics at this stage, so I hoped someone who wanted to port to standard would port this feature.
It wasn't what I expected, but I think it's good that a similar feature was implemented in standard.

Since extended has a class classification for passengers and mail, I planned to divide it into a large classification and a small classification.
That is like:
[ All pas ] [pas class1] [pas class2] [pas class3]...
[ All mail ] [mail class1] [mail class2]...
[ All freight ] [freight catg1] [freight catg2] [freight catg3]...
However, this made me think about the future of the line window. I think the best way would be lines a independent windows, opened from the line management window. Then oen could compare statistics from each line side by side and it would make the window also smaller.
Yes, I'm still wondering what is best layout and I haven't decided how to reserve the above button area. (´・ω・`)