Kesediaan Membayar Perusahaan Tambang Batubara Terhadap Pelayanan Pengangkutan Dengan Rel Kereta Api : Studi Kasus Purukcahu Bangkuang, Provinsi Kalimantan Tengah = Willingness to pay coal companies with transportation services railway : case study Purukcahu - Bangkuang, Central Kalimantan province