Train Buddy Dilemma

Dear CJ

I recently started reading your website and actually worked my way back from the first post to your most recent posts and I noticed along the way that you've made "train friends" where it appears Charlotte is the first person you hooked up with.
Like you, I also have train friends and only one of the three people I sit with is someone I socialize with on an "off the train" level as we get along well, but the other two women I sit with don't like her. I feel sometimes they sit with us because they enjoy my company and don't want to be rude. I also feel they wouldn't move just to be petty and I'll get to that in a bit.
I think I need to give these women names so that I can identify who is who.
The woman I socialize with is Kate. The other women are Anne and Debbie. These aren't real names and the situation I'm about to describe is not the real situation but is parallel to it.
We've all been train buddies for two years.
In September Kate got engaged. Last week she handed me, on the train, in front of Anne and Debbie, an invitation to her wedding in May and asked all 3 of us to email her our mailing addresses so her maid of honour has a complete shower list.
She then went on to tell Anne and Debbie she'd invite them to the wedding but the guest list is at the max and that she really wanted me there because we're more than train friends. This was news to me. I like her but we hook up for lunch occasionally and nothing really more. I don't even have her home phone number, just work and cell.
What I don't understand is why she invited me in the first place. I don't want to go to her shower or wedding only because I am a single mom and money is always tight. She knows this and so has put me in a position to decline both.
Second, I thought it was insensitive of her to explain to Debbie and Anne why they weren't invited to her wedding and I don't feel she should include them in the shower. She's made this very personal, don't you agree?
Although I like Kate very much and we get together for lunch, for me, train friendships are like work friendships. These are casual acquaintances with boundaries.
You seem to have quite the following and I'm interested in what other women think of my situation.
Recently, Kate also disclosed that she doesn't like sitting with Anne and Debbie as she only wants to talk to me and has asked me to consider riding in a different car. She says she feels like Debbie and Anne are always judging her, where those two have disclosed to me they feel that Kate doesn't care about their lives and talks over them on purpose.
I like everyone equally and don't want to choose. I also think it's mean to just stop talking to people because of another person's shortcomings.
As you can read, this is a situation concerning several situations. It was much easier to be train friends when it wasn't so personal and I somewhat feel Kate should have kept her wedding/shower plans "off the train".

Sent from my BlackBerry device on the Rogers Wireless Network from Mrs. Y.