## 1 Introduction  
Ballasted tracks are commonly used because their initial construction cost is low and ground settlement can be managed flexibly; however, settlement may occur owing to continuous train load, which may cause differential settlement. Maintenance work, such as tamping, should be performed regularly to prevent such phenomena. However, repeated maintenance works can result in the deterioration of ballast [1]. When maintenance work is performed for a section with differential settlement, the severity of differential settlement is reduced while increasing the abrasion loss of the ballast aggregates, thereby ultimately reducing the ballast resistance against settlement [2; 3; 4]. Afterwards, differential settlement occurs again when a train load is applied, and performing maintenance work to reduce the differential settlement will further increase the abrasion loss of gravels. As this phenomenon is repeated, the abrasion loss of gravels increases, and the period for maintenance works shortens; hence, ballast cleaning, i.e., ballast renewal will be required. A kind of ballast aggregates in Korea is rhyolitic tuff. In Korea railroad corporation standards [5], there are some requirements for ballast properties. Fresh ballast size is between 22.4-63 mm and abrasion loss is under 25%. The railroad maintenance guidelines of the Korea National Railway [6] define the following four criteria to perform ballast cleaning: when the soil contents in the ballast for a normal railroad is 25% or greater; when the ballast layer has 20% or higher of fine materials passing 22.4 mm sieve; when high fouling level and poor drainage; and when ballast layer has poor resistance against settlement due to abrasion. Among these criteria, the objective criterion for high-speed railways without considering the subjective opinions of workers is the second one; fine materials passing For railways in operation, regular maintenance works can be performed at night only when trains are not operating. Owing to such a time constraint and difficulties with sampling ballast gravels, observing the condition of ballast gravels based on this criterion has limitations. Therefore, studies have been conducted with regard to measuring the abrasion of gravels based on image analysis methods. An aggregate imaging system involves the analysis of images using black and white images which are taken with back-lighting, and gray images which are taken with top-lighting [7,8]. The University of Illinois Aggregate Image Analyzer used three cameras to capture the front, top, and side views to reconstruct the 3-D shape of aggregates [9,10]. [PERSON] et al. deduced a correlation between the abrasion loss of gravels and the shape index by performing 2-D digital image analysis and applied the correlation at fields to prove the relationship among the abrasion loss, shape index, and maintenance work [11]. Furthermore, image analysis based on a 3-D image has been studied. [PERSON] et al. proposed wavelet-based 3-D particle descriptors to characterize stone aggregate morphology using a 3-D laser scanner [12]. [PERSON] analyzed aggregates image with a 3-D laser scanning technique and analysis method based on the fast Fourier and power spectrum analyses [13]. [PERSON] acquired 3-D particle images through X-ray tomography to characterize concrete aggregate [14]. [PERSON] et al. used point clouds from 3-D digitization of the particle to characterize the shape of ballast [15]. Relying on only 2-D images to analyze the characteristics of gravels used can be rather inaccurate, because the characteristics of gravels may vary significantly depending on the axis from which the images are taken [16]. However, 3-D image analysis appears inappropriate for application to actual practice as special equipment, such as an X-ray or a 3-D laser scanner, for taking 3-D images and relatively more time than 2-D image analysis are required. In this study, we improve the image analysis method developed by [PERSON] et al. [11] to increase the recognition rate of gravel aggregates.